Spot Test Kits For Detecting

Lead in Household Paint:

A Laboratory Evaluation

Walter J. Rossiter, Jr.

Mark G. Vangel

Mary E. McKnight

Gary Dewalt

Prepared for:

 U.S. Department of Housing and Urban Development

Office of Lead Hazard Control

NISTIR 6398
NISTIR 6398

Spot Test Kits For Detecting

Lead in Household Paint:

A Laboratory Evaluation

Walter J. Rossiter, Jr.*

Mark G. Vangel**

Mary E. McKnight*

*Building and Fire Research Laboratory

**Information Technology Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-8621

Gary Dewalt

QuanTech

Rosslyn, VA 22209-1607

May 2000

U.S. Department of Commerce

William M. Daley, Secretary

Technology Administration

Dr. Cheryl L. Shavers, Under Secretary of 

Commerce for Technology

National Institute of Standards and Technology

Raymond G. Kammer, Director

Prepared for:

U.S. Department of Housing and Urban Development

Andrew M. Cuomo, Secretary

Office of Lead Hazard Control

David E. Jacobs, Director
iii

ABSTRACT

A laboratory study was conducted to determine the reliability of spot test kits for detecting the 

presence of lead in household paint when tests were conducted by certified lead inspectors or risk 

assessors. Reagent solutions were applied to paint specimens and, subsequently, the specimens were 

observed for characteristic color change. For the study, four test kits were based on the reaction of 

lead ion with sulfide ion to produce a gray or black color, whereas four others were based on the 

reaction of lead ion with rhodizonate ion to give a pink or red color. These eight kits were used in 

an experiment investigating the effect of lead level, lead pigment type, operator, paint-film substrate, 

overlayer paint type, and overlayer paint thickness. Test samples, prepared using either a white lead 

(i.e., basic lead carbonate) or a lead chromate pigment, had ten lead levels ranging from 0 mg/cm2 to 

3.5 mg/cm2. Five operators were trained according to test protocols based on each kit 

manufacturer’s instructions. The study showed that the spot test kits gave positive results at lead 

levels less than 1 mg/cm2. Consequently, a positive response could not be relied on to indicate the 

presence of lead-based paint, which is defined as paint having lead levels equal to, or greater than, 

1 mg/cm2. This finding is consistent with the results of past field studies. A criterion against which a 

spot test kit may be considered as acceptable for use as a negative screen (i.e., a test for which a 

negative result indicates a low probability of lead $ 1 mg/cm2) for the presence of lead-based paint 

was proposed. This criterion is: Upon evaluation of spot test kit response, the probability of a 

negative response (with 95 % confidence) at a lead level of 1 mg/cm2 is # 5%. Equivalently, the lead 

level at which there is a 95 % probability of a positive response (with 95 % confidence) should be #

1 mg/cm2. The type of lead pigment had a significant effect on the spot test kit response. For white 

lead specimens, six kits—three sulfide-based and three rhodizonate-based—gave low percents of 

false negatives (# 2 %) and met the proposed criterion for acceptance as a negative screen for leadbased paint. For lead chromate specimens, three of these six kits—two sulfide-based and one 

rhodizonate-based—also had low percents of false negatives (# 2 %) and met the proposed 

acceptance criterion. The other factors—overlayer type, overlayer thickness, operator, and 

substrate—did not generally show significant effects in cases where the spot test kits appeared to be 

candidates for use as negative screens for lead-based paint. Finally, the study results lead to the 

suggestion that an evaluation of spot test kit response should afford a low percent of positive results 

at the 0 mg/cm2 lead level because, in practice, false-positives may needlessly spur test kit users into 

taking further, but unnecessary, investigative action for the presence of lead.

Key words: building technology; detection; kit response; lead-based paint; lead level; lead chromate; 

operator effect; spot test kits; testing; white lead 
iv

TABLE OF CONTENTS

 

Page

ABSTRACT................................................................................................................................... iii

1. INTRODUCTION......................................................................................................................1

1.1 Background....................................................................................................................1

1.2 Objective and Scope of the Study ...................................................................................3

2. EXPERIMENTAL DESIGN ......................................................................................................3

2.1 Factors Affecting Spot Test Kit Response.......................................................................3

2.2 Fractional Factorial Design .............................................................................................8

3. SPOT TEST KITS IN THE STUDY ........................................................................................ 11

3.1 Kits Having Multiple Test Procedures........................................................................... 12

4. EXPERIMENTAL ................................................................................................................... 13

4.1 Sample Preparation....................................................................................................... 13

4.1.1 Leaded Paint.................................................................................................. 13

4.1.2 Leaded-Paint Films ........................................................................................ 14

4.1.3 Coated Test Panels......................................................................................... 15

4.1.4 Surface Test Grid and Panel Identification...................................................... 16

4.1.5 Leaded-Paint Chips........................................................................................ 16

4.1.6 Selection of Lead Levels................................................................................ 16

4.1.7 Laboratory Lead Analysis............................................................................... 16

4.2 Testing and Data Recording ......................................................................................... 17

5. RESULTS AND DISCUSSION ............................................................................................... 24

5.1 False Negatives and False Positives............................................................................... 24

5.1.1 Effect of STK4 Test Procedure ...................................................................... 27

5.2 Factors Affecting Spot Test Kit Response..................................................................... 28

5.2.1 Method of Analysis........................................................................................ 28

5.2.2 Effect of Overlayer Type and Overlayer Thickness......................................... 28

5.2.3 Effect of Lead Pigment Type.......................................................................... 29

5.2.4 Effect of Operator.......................................................................................... 30

5.3 Modeling the Probability of Spot Test Kit Response ..................................................... 31

5.3.1 Ideal Test Kit Performance for Detecting Lead-Based Paint ........................... 45

5.3.2 Effect of Spot Test Kit................................................................................... 46

5.3.3 Effect of Lead Pigment Type.......................................................................... 46

5.3.4 Effect of Spot Test Kit Procedure .................................................................. 46

5.3.5 Effect of Operator.......................................................................................... 47

5.3.6 Effect of Substrate ......................................................................................... 47

5.3.7 Comparison of NIST Laboratory Data with EPA/HUD Field Data ................. 48

5.4 Upper Confidence Limits on the Lead Level Having a 95 % Probability of a 

 Positive Response......................................................................................................... 55

6. SUMMARY AND CONCLUSIONS ........................................................................................ 57
v

7. ACKNOWLEDGMENTS......................................................................................................... 59

8. REFERENCES......................................................................................................................... 59

APPENDIX A. ADDITIONAL TABULATIONS OF FALSE NEGATIVES AND FALSE

 POSITIVES ....................................................................................................... A1

APPENDIX B. TABULATIONS OF THE RESULTS OF MODELING TEST KIT 

RESPONSE VERSUS LEAD LEVEL ............................................................... B1

LIST OF TABLES

Page

Table 1. Factors varied during the study..........................................................................................4

Table 2. Spot test kits and methods of use.......................................................................................6

Table 3. Numbers of test kits, lead levels, operators, and lead pigment types selected in

the experimental design for kits used to test paint films adhered to substrates ....................8

Table 4. Combinations of substrate, overlayer type, and overlayer thickness selected in

the experimental design for kits used to test paint films adhered to substrates ...................8

Table 5A. Experimental design combinations for white lead including four controls.........................9

Table 5B. Experimental design combinations for lead chromate.................................................... 10

Table 6. Results of sodium sulfide analyses of sulfide-based spot test kits...................................... 12

Table 7. Lead levels prepared in the study ..................................................................................... 13

Table 8A. Targeted and mean lead levels for the test panels having white lead

and non-reactive substrates........................................................................................... 18

Table 8B. Targeted and mean lead levels for the test panels having white lead

and reactive substrates.................................................................................................. 19

Table 8C. Targeted and mean lead levels for the test panels having lead

chromate and non-reactive substrates ........................................................................... 20

Table 8D. Targeted and mean lead levels for the test panels having lead

chromate and reactive substrates .................................................................................. 21

Table 9A. Targeted and mean lead levels for chips with white lead................................................ 22

Table 9B. Targeted and mean lead levels for chips with lead chromate .......................................... 23
vi

Table 10A. False negatives and false positives at a lead level of 0 mg/cm2..................................... 25

Table 10B. False negatives and false positives at a lead level of 1.0 mg/cm2................................... 27

Table 11. F-tests of the significance of overlayer type and overlayer thickness............................... 29

Table 12. F-tests of the significance of lead pigment type .............................................................. 30

Table 13. F-tests of the significance of operator ............................................................................ 31

Table 14. Lead level corresponding to a 95 % probability of a positive response for 

an Aaverage@ operator .................................................................................................... 56

Table A1. False negatives and false positives at a lead level of 0.5 mg/cm2................................... A1

Table A2. False negatives and false positives at a lead level of 0.7 mg/cm2................................... A2

LIST OF FIGURES

Page

Figure 1. Probability of a Positive Response Versus Lead Level for Spot Test Kit 1 (STK1) 

for Each Operator. (The filled circles represent the proportions of positive 

responses at a given lead level, and the error bars are the associated 95 % binomial 

confidence intervals. The two horizontal dashed lines represent the 0.5 and 0.95 

probabilities of a positive response. Circles are the 0.95 probability point for each 

operator; they are repeated above the horizontal axis for clarity.) ...........................33

Figure 2. Probability of a Positive Response Versus Lead Level for Spot Test Kit 2 (STK2) 

for Each Operator. (The filled circles represent the proportions of positive 

responses at a given lead level, and the error bars are the associated 95 % binomial 

confidence intervals. The two horizontal dashed lines represent the 0.5 and 0.95 

probabilities of a positive response. Circles are the 0.95 probability point for each 

operator; they are repeated above the horizontal axis for clarity.) ...........................34

Figure 3. Probability of a Positive Response Versus Lead Level for Spot Test Kit 3 (STK3) 

for Each Operator. (The filled circles represent the proportions of positive 

responses at a given lead level, and the error bars are the associated 95 % binomial 

confidence intervals. The two horizontal dashed lines represent the 0.5 and 0.95 

probabilities of a positive response. Circles are the 0.95 probability point for each 

operator; they are repeated above the horizontal axis for clarity.) ...........................35

Figure 4A. Probability of a Positive Response Versus Lead Level for Spot Test Kit 4 for Each

Operator; Data Are For the STK4a Procedure. (The filled circles represent the

proportions of positive responses at a given lead level, and the error bars are the

associated 95 % binomial confidence intervals. The two horizontal dashed lines 

represent the 0.5 and 0.95 probabilities of a positive response. Circles are the 0.95

probability point for each operator; they are repeated above the horizontal axis for

clarity.)........................................................................................................................ 36
vii
viii

Figure 4B. Probability of a Positive Response Versus Lead Level for Spot Test Kit 4 for Each 

Operator; Data Are For the STK4b Procedure. (The filled circles represent the 

proportions of positive responses at a given lead level, and the error bars are the 

associated 95 % binomial confidence intervals. The two horizontal dashed lines 

represent the 0.5 and 0.95 probabilities of a positive response. Circles are the 0.95 

probability point for each operator; they are repeated above the horizontal axis for 

clarity.)........................................................................................................................ 37

Figure 4C. Probability of a Positive Response Versus Lead Level for Spot Test Kit 4 for Each 

Operator; Data Are For the STK4c Procedure. (The filled circles represent the 

proportions of positive responses at a given lead level, and the error bars are the 

associated 95 % binomial confidence intervals. The two horizontal dashed lines 

represent the 0.5 and 0.95 probabilities of a positive response. Circles are the 0.95 

probability point for each operator; they are repeated above the horizontal axis for 

clarity.)........................................................................................................................ 38

Figure 5. Probability of a Positive Response Versus Lead Level for Spot Test Kit 5 (STK5) 

for Each Operator. (The filled circles represent the proportions of positive 

responses at a given lead level, and the error bars are the associated 95 % binomial 

confidence intervals. The two horizontal dashed lines represent the 0.5 and 0.95 

probabilities of a positive response. Circles are the 0.95 probability point for each 

operator; they are repeated above the horizontal axis for clarity.)............................39

Figure 6. Probability of a Positive Response Versus Lead Level for Spot Test Kit 6 (STK6) 

for Each Operator. (The filled circles represent the proportions of positive 

responses at a given lead level, and the error bars are the associated 95 % binomial 

confidence intervals. The two horizontal dashed lines represent the 0.5 and 0.95 

probabilities of a positive response. Circles are the 0.95 probability point for each 

operator; they are repeated above the horizontal axis for clarity.)............................40

Figure 7A. Probability of a Positive Response Versus Lead Level for Spot Test Kit 7 for Each 

Operator; Data Are For the STK7a Procedure. (The filled circles represent the 

proportions of positive responses at a given lead level, and the error bars are the 

associated 95 % binomial confidence intervals. The two horizontal dashed lines 

represent the 0.5 and 0.95 probabilities of a positive response. Circles are the 0.95 

probability point for each operator; they are repeated above the horizontal axis for 

clarity.)........................................................................................................................ 41

Figure 7B. Probability of a Positive Response Versus Lead Level for Spot Test Kit 7 for Each 

Operator; Data Are For the STK7b Procedure. (The filled circles represent the 

proportions of positive responses at a given lead level, and the error bars are the 

associated 95 % binomial confidence intervals. The two horizontal dashed lines 

represent the 0.5 and 0.95 probabilities of a positive response. Circles are the 0.95 

probability point for each operator; they are repeated above the horizontal axis for 

clarity.)........................................................................................................................ 42
ix

Figure 8A. Probability of a Positive Response Versus Lead Level for Spot Test Kit 8 for Each 

Operator; Data Are For the STK8a Procedure. (The filled circles represent the 

proportions of positive responses at a given lead level, and the error bars are the 

associated 95 % binomial confidence intervals. The two horizontal dashed lines 

represent the 0.5 and 0.95 probabilities of a positive response. Circles are the 0.95 

probability point for each operator; they are repeated above the horizontal axis for 

clarity.) ....................................................................................................................... 43

Figure 8B. Probability of a Positive Response Versus Lead Level for Spot Test Kit 8 for Each 

Operator; Data Are For the STK8b Procedure. (The filled circles represent the 

proportions of positive responses at a given lead level, and the error bars are the 

associated 95 % binomial confidence intervals. The two horizontal dashed lines 

represent the 0.5 and 0.95 probabilities of a positive response. Circles are the 0.95 

probability point for each operator; they are repeated above the horizontal axis for 

clarity.) ....................................................................................................................... 44

Figure 9. Example of the Ideal Performance of a Spot Test Kit for Determining Lead-Based 

Paint. ............................................................................................................................. 45

Figure 10. Comparison of the Results of the NIST Laboratory Data With Those of the 

EPA/HUD Field Data [8] for Spot Test Kit 3. (The solid and dashed curves 

represent the NIST and EPA/HUD data, respectively.) ................................................ 50

Figure 11. Comparison of the Results of the NIST Laboratory Data With Those of the 

EPA/HUD Field Data [8] for Spot Test Kit 4. The Comparison was Made for the

STK4b Procedure. (The solid and dashed curves represent the NIST and 

EPA/HUD data, respectively.) ..................................................................................... 51

Figure 12. Comparison of the Results of the NIST Laboratory Data With Those of the 

EPA/HUD Field Data [8] for Spot Test Kit 6. The EPA/HUD Study did not 

Include Data for a Reactive Substrate. (The solid and dashed curves represent 

the NIST and EPA/HUD data, respectively.)................................................................ 52

Figure 13. Comparison of the Results of the NIST Laboratory Data With Those of the 

EPA/HUD Field Data [8] for Spot Test Kit 7. The Comparison was Made for the 

STK7b Procedure. (The solid and dashed curves represent the NIST and 

EPA/HUD data, respectively.) ..................................................................................... 53

Figure 14. Comparison of the Results of the NIST Laboratory Data With Those of the 

EPA/HUD Field Data [8] for Spot Test Kit 8. The Comparison was Made for the 

STK8b Procedure. (The solid and dashed curves represent the NIST and 

EPA/HUD data, respectively.) ..................................................................................... 54
1

1. INTRODUCTION

1.1 Background

According to a recent report by the President’s Task Force on Environmental Health Risks and 

Safety Risks to Children, approximately 24 million U.S. dwellings were at risk for lead-based paint 

hazards in 1999 [1]. As defined in Public Law 102-550, Residential Lead-Based Paint Hazard 

Reduction Act of 1992, the term lead-based paint means paint or other surface coatings that contain 

lead at contents that Aequal or exceed a level of 1.0 milligram per centimeter squared or 0.5 percent 

by weight@ [2]. A definition is also given in the Guidelines for the Evaluation and Control of LeadBased Paint Hazards in Housing, a document often called the HUD Guidelines [3]. The accurate 

and efficient identification of lead-based paint in housing is important to the Federal government. 

For example, identification of lead-based paint in most pre-1978 Atarget@ housing∗ requires disclosure 

of that information, if available, to the owner, prospective purchasers, or tenants (42 U.S.C. 4852d, 

24 CFR 35.80-98). Also, in certain target housing receiving financial assistance from HUD, or being 

sold by the Federal government, identification of lead-based paint results in requirements for 

lead-based paint hazard evaluation and/or control (42 U.S.C. 4822, 24 CFR 35.1-1355).

As far back as the early 1970s, spot tests were introduced as relatively nonintrusive, potentially costsaving, qualitative methods for determining the presence or absence of lead-based paint on-site [4,5]. 

A spot test involves Athe application of reagent solution to a prepared dry paint film sample, paint 

chip, paint powder, or painted surface and the subsequent observation for the presence or absence of 

the characteristic color change@ [6]. Presently, two types of spot tests are used for detecting lead in 

paint, and prepackaged kits are commercially available from a number of suppliers [7,8]. One type is 

based on the reaction of rhodizonate ion with lead II ion; this reaction produces in acidic solution a 

color change from yellow-orange to pink or red. The other is based on the reaction of sulfide ion 

with lead II ion; here the color change is from clear to gray or black.∗∗ In performing a spot test, the 

basic procedure is to cut a notch through, or to abrade the surface of, the paint film, then to place the 

reagent solution on that location, and finally to observe qualitatively whether a characteristic color 

change occurs. Variations to this general procedure include placing the reagent solution on paint 

chips, and mixing paint chips in a leaching solution which is, in turn, tested with the reagent. The 

American Society for Testing and Materials (ASTM) has issued two standards associated with the 

use of spot tests: ASTM E 1753, Practice for Use of Qualitative Chemical Spot Test Kits for 

Detection of Lead in Dry Paint Films [6] and ASTM E 1828, Guide for Evaluating the 

Performance Characteristics of Qualitative Chemical Spot Test Kits for Lead in Paint [9].

Potential advantages to using spot tests over other methods of identifying lead-based paint include 

that spot test methods: are inexpensive and rapid, may require minimal operator technique, and may 

respond to microgram levels of analyte [7]. The major barrier to the acceptance of spot tests has 

been indications that they may be unreliable for identifying lead-based paint [8,10,11]. For example, 

 

*The definition of target housing in the HUD Guidelines [3] is: Any residential unit constructed before 1978, 

except dwellings that do not contain bedrooms or dwellings that were developed specifically for the elderly or persons 

with disabilitiesCunless a child younger than 6 resides or is expected to reside in the dwelling. In the case of 

jurisdictions that banned the sale or use of lead-based paint before 1978, the Secretary of HUD may designate an 

earlier date for defining target housing.

**The two types of spot tests are referred to as rhodizonate and sulfide in this report.
2

a 1997 EPA report [11] indicated the following concerns: spot tests are subject to positive results in 

which lead is indicated when it is not present at significant levels; spot test reagents may

not solubilize the lead resulting in false negative∗ results indicating the absence of lead when it is 

present in significant levels; and spot tests do not provide a reliable transition from negative response 

to positive response at the Federal level at which a paint is classified as lead-based (i.e., 1 mg/cm2).

Many studies [7,8,11-20] have been conducted in the laboratory and field to evaluate the 

performance of spot tests. In one of the earliest studies in the late 1970s, Vind, Mathews, 

Alumbaugh, and Hamilton [12] reported that they were able to forego laboratory analysis of 150 out 

of 250 field paint samples because the spot tests conducted with sodium sulfide reagent were 

considered Aunquestionably negative.@ In the late 80s, in a study incorporating about 70 samples, 

McKnight, Byrd, Roberts, and Lagergren [13] suggested that spot tests conducted by experienced 

technicians may be useful in indicating the presence of lead at or near the lead-based paint level of 

1 mg/cm2, but recommended further evaluation. More recently, Ashley, Hunter, Tait, Dozier, 

Seaman, and Berry [20] concluded from a study of about 200 paint films using a rhodizonate spot 

test kit that Ain-situ testing of lead in paint by [....] chemical spot test kits can be used for screening 

(i.e., qualitative) purposes.@ In contrast to these examples, a 1995 field study jointly funded by EPA 

and HUD [8] concluded that test kits should not be used for lead paint testing because they Acannot 

determine the extent of lead-based paint in a home...@ This EPA/HUD study included the most 

extensive field testing conducted with spot test kits. It incorporated six kits (4 rhodizonate and 2 

sulfide), five of which were each used on about 1300 test locations. The testing was conducted by 

individuals selected to represent typical homeowners who might purchase kits for personal use. The 

conclusion not to use test kits for identifying lead-based paint was based, in part, on the finding that 

they varied widely in performance and that none demonstrated low percents of both false positive 

and false negative results in comparison with the Federal level of 1 mg/cm2 at which a paint is 

classified as lead-based. Similarly, in a narrower 1997 field study involving two test kits 

(1 rhodizonate and 1 sulfide) and 120 test locations, Reames, Brumis, Lance, and Schwartzberg [19] 

recommended that spot test kits not be used for lead-based paint inspection. In this case, the authors 

found that, although low percents of false negatives were achieved at the 1 mg/cm2 level, both test 

kits had high percents of false positives. 

Spot tests are not currently used in Federal programs for assessing the presence or absence of 

lead-based paint in housing. The present study is intended to provide further evaluation of spot test 

kit performance and, in particular, when they are used by well-trained operators in a well-controlled, 

systematic laboratory study. The results would help to support future decisions regarding their 

possible use. If the results indicate that spot tests are reliable under well-controlled, laboratory 

conditions, follow-up studies might be justified to pinpoint reasons why some field studies have 

found spot tests to be unreliable. Conversely, if the results of a well-controlled laboratory 

experiment found spot tests to be unreliable, then further field studies would not be appropriate. 

Thus, HUD requested that the National Institute of Standards and Technology (NIST) conduct a 

well-controlled, systematic laboratory study on the reliability of spot tests for detecting lead in 

household paint.

 

*

A false negative has been defined as a test result that is negative for lead, but the true value is above the 

selected lead level [8]. Conversely, a false positive is a test result that is positive for lead, but the true value is below 

the selected lead level.
3

1.2 Objective and Scope of the Study

This report presents the results of the HUD-sponsored study to determine the reliability of spot test 

kits for detecting the presence of lead in household paints when tests are conducted in the laboratory 

by certified lead inspectors or risk assessors. Seven factors that may affect the performance of spot 

tests were varied according to a predetermined experimental design. Descriptions of these seven 

factorsCspot test kit, lead level*, lead pigment type, operator, paint film substrate, overlayer type, 

and overlayer thicknessCare given in Table 1 with a comment as to why each factor was included. 

Table 2 provides information on the eight spot test kits, designated STK1 through STK8. The test 

samples were prepared in the laboratory, which allowed control of the design factors, and made 

possible a balanced statistical design. During the testing, the response of each test kit (i.e., negative 

or positive) for each specimen was recorded. The data were statistically analyzed (1) to determine 

the effects of the factors incorporated in the design (Table 1) or interactions among them, and (2) to 

quantify the probability of lead detection through use of each spot test kit.

HUD not only sponsored this study at NIST, but also a complementary study conducted by 

QuanTech, Rosslyn, VA. The objectives of the QuanTech study were to generate a guide for using 

spot test kits for detecting the presence of lead-based paint in residential housing, and to validate the 

use of laboratory-prepared test samples as surrogates for field samples. Because the two studies 

were complementary, NIST and QuanTech performed much of the research cooperatively. Only one 

set of laboratory test samples was prepared for both the NIST and QuanTech studies, and most of 

the spot test kits used were common to both. QuanTech research staff conducted its laboratory spot 

tests in the NIST laboratories, and the results are included in the present report as Op4 and Op5 

data. Note that the complementary QuanTech study produced two reports, The Use of 

Manufactured Samples for Evaluating Spot Test Kits for Detecting Lead in Household Paints [21], 

and Guidance for the Evaluation of Spot Test Kits for Detecting the Presence of Lead in Household 

Paints [22].

2. EXPERIMENTAL DESIGN

2.1 Factors Affecting Spot Test Kit Response

Factors other than lead level are variable in practice and may affect spot test response. For example, 

for both rhodizonate- and sulfide-based spot tests, more than one kit is available, and human 

involvement in subjectively judging response may affect the results. Different types of lead pigments 

have historically been used in paint production and differences in their solubilities can also affect 

response. Also, depending on the age of a residence and how often it has been painted, the type and 

thickness of paint layers covering lead-containing paint films may affect the results. The factors 

included in the experimental design are described in Table 1.

A key consideration is the lead level at which the spot test kit response changes from negative to 

positive; consequently, 10 lead levels were incorporated in the design. Equally important is 

recognition that different kits used by various operators may have varying response for a given test 

sample and, thus, eight test kits and five operators were included. Two levels each were chosen for 

lead pigment type, film substrate, overlayer type, and overlayer thickness. In these cases, the levels 

 

*

HUD prefers determining the amount of lead in a paint film on the basis of area content (i.e., mg/cm2) as 

opposed to mass concentration. Hence, the experimental design of this study is based on area content, which is 

referred to as Alead level@ in this report.
4

were selected to be representative of real-world practiceCfor example, latex and oil-based paint 

overlayersCor to bracket the extremes of what is likely to be encountered in practiceCfor example, 

the solubility of the lead pigment, and thin and thick overlayers (Table 1).

Table 1. Factors varied during the study

Factor Description Comment

Spot Test 

Kit

Eight spot test kits were included:

 $Spot Test Kit 1 (STK1) 

 $Spot Test Kit 2 (STK2) 

 $Spot Test Kit 3 (STK3) 

 $Spot Test Kit 4 (STK4) 

 $Spot Test Kit 5 (STK5) 

 $Spot Test Kit 6 (STK6) 

 $Spot Test Kit 7 (STK7) 

 $Spot Test Kit 8 (STK8) 

In practice, spot test kits are available from a number of 

suppliers. Four rhodizonate and four sulfide kits were included 

in the study. For both types, the selected kits covered the range 

of kits used in practice for examining the entire paint film (by 

notching, coring, or chipping), and not just the surface of the 

paint film. Seven of the eight kits were commercial products. 

STK8 was a kit available to professional inspectors and risk 

assessors through a state government laboratory. STK1, STK2, 

STK7, and STK8 were sulfide kits; STK3 through STK6 were 

rhodizonate kits. STK2 was sold to test only chips and, 

consequently, the paint film specimens tested with STK2 were 

not adhered on a substrate.

Lead 

Level

Ten lead levels were chosen for 

each lead pigment type. For both 

leads, the targeted range was from 

no lead added to 3.5 mg/cm2. The 

targeted values (in mg/cm2 ) were 

as follows:

white lead lead chromate

 $0 $0

 $0.1 $0.5

 $0.2 $0.7

 $0.3 $1.0

 $0.4 $1.2

 $0.5 $1.6

 $0.7 $1.8

 $1.0 $2.0

 $1.6 $2.5

 $3.5 $3.5

In practice, lead in paint films in existing houses varies from 

essentially none (i.e., lead was not purposely added to the 

paint) to substantial. For example, mean lead levels in the 

range of 2 mg/cm2 to 3 mg/cm2 have been measured in field 

studies [8,19]. The ranges of lead level in the present study 

were consistent with those found in the field. The ten values 

for each lead pigment type were chosen on the basis of a 

preliminary experiment (Section 4.1.6), and more high levels 

were taken for the less soluble lead chromate pigment.

Note 1: A lead level of 0 mg/cm2 is the designation assigned to 

test panels for which lead was not added to the paint films. 

Measurements showed that the lead levels of these panels was 

< 0.009 mg/cm2 (Section 4.1.7).

Note 2: A distinction between white lead and lead chromate for 

specimens having a 0 mg/cm2 lead level is artificial because 

such specimens do not contain lead. Nevertheless, the 

distinction is maintained in the discussions because of the 

balance of the experimental design.

Operator Five operators were included:

 $Operator 1 (Op1)

 $Operator 2 (Op2)

 $Operator 3 (Op3)

 $Operator 4 (Op4)

 $Operator 5 (Op5)

In practice, spot test kits are available to many people whose 

abilities may vary considerably. The operator factor addressed 

the effect of Athe human element@ on test kit response. Initial 

planning for the experimental program considered selecting 

three operators (Op1 through Op3), who were required to be 

either certified lead inspectors or risk assessors. During 

experimental design, two additional operators (Op4 and Op5) 

became available for participation. They did not conduct tests 

with STK2 and STK8. Because their partial participation 

provided additional data for STK1, and STK3 through STK7, 

they were included in the test program. Op4 and Op5 were

QuanTech research professionals having experience in the use 

of spot test kits. Both had passed an EPA lead-inspector 

training course, although neither was certified. Before 

beginning the test program, Op1, Op2, and Op3 were examined 

at the NIST Health Unit for red colorblindness. Op4 and Op5 

had been checked during a previous spot test kit study. None of 

the five operators were red colorblind. 
5

Table 1. Factors varied during the study (cont.)

Factor Description Comment

Lead 

Pigment

Type

Two lead pigment types were 

included:

 $relatively soluble (white lead)

 $relatively insoluble (lead 

chromate)

In practice, a number of lead-based pigments have historically 

been used in paint production. These include basic lead 

carbonate, basic lead sulfate, lead silicate, chrome yellows and 

oranges (lead chromate combined with lead sulfate, lead 

carbonate, and lead phosphate to obtain different hues), chrome 

greens (chrome yellow and iron blue), molybdate orange (lead 

molybdate and lead sulfate) and red lead (Pb3O4) [23]. The 

most common pigment was basic lead carbonate, whose 

composition is approximately 2PbCO3CPb(OH)2 [24-26]. This 

pigment is often referred to as Awhite lead,@ which is the term 

used in this report. White lead is relatively soluble compared to 

other lead pigments. In the study, white lead and lead 

chromate were selected to cover the solubility range of lead 

pigments. Although used considerably less than white lead, 

lead chromate was a basic pigment for some green, red, orange, 

and yellow house paints through the mid-1960s [26].

Substrate Two types of substrates, a nonreactive (NR) and a reactive (R), 

substrate, were included for spot 

tests conducted on panel samples:

 $sulfide: wood (NR)

 steel (R)

 $rhodizonate: wood (NR)

 plaster (R)

In practice, lead-based paint has been applied to may different 

substrates. The reactivity, or nonreactivity, of the substrate 

may affect spot test response. For example, steel and plaster 

substrates can affect sulfide and rhodizonate spot tests, 

respectively [6]. The substrate factor investigated the effect of 

substrate reactivity on test kit response. It was beyond the 

scope of the study to include all three substrates for all kits, as 

steel was considered to be non-reactive for rhodizonate, and 

plaster was considered non-reactive for sulfide. 

Overlayer 

Type

Two overlayer types were included:

 $latex paint

 $oil-based paint

In practice, lead-based paint is quite likely to be covered with 

additional layers of paint. These overlayers may be latex or oilbased paint. This factor was included to determine if the type 

of overlayer paint affects spot test response.

Overlayer 

Thickness

Two overlayer thicknesses were 

included:

For panels on substrates:

 $thin: about 0.13 mm to 0.28 mm

 (0.005 in to 

0.011 in)

 $thick: about 0.75 mm to 1.4 mm

 (0.030 in to 

0.055 in)

In practice, spot test kits have been used to test paint layers 

having varying thicknesses. For example, Reames et al. [19] 

reported thickness values ranging from about 0.1 mm to 

1.5 mm (0.004 in to 0.060 in). This factor was included to 

determine if overlayer thickness affects spot test response. The 

thicknesses of the thin and thick overlayers were selected to 

cover the range of overlayer thicknesses measured in the field.
6

Table 2. Spot test kits and methods of use

Spot 

Test

Kit

Spot Test

Type

Characteristic

Color for Leada General Method of Usea

STK1 Sulfide Gray to black $Cut a notch through the paint exposing all layers; illuminate where 

necessary and examine with a magnifying glass to insure that all layers 

are exposed.

$Place a small drop of the reagent solution on one half of the notch; this 

allows a comparison with the untreated section of the notch.

$Observe test surface for characteristic color.

$A vial of lead acetate paper is supplied with the kit to check reagent.

STK2 Sulfide Brown or black $Place about 1 teaspoon of chips in a test tube (provided with kit).

$Add an equal amount of vinegar to the test tube.

$Cap test tube and let stand for 24 h.

$Stir using plastic pipette; then allow solids to settle.

$Add up to 10 drops of reagent to the test tube.

$Observe test tube for characteristic color.

STK3 Rhodizonate Pink to dark 

purple

$Remove all dust and dirt.

$Cut or scrap through all paint layers to expose bare surface .

$Moisten pad, which is impregnated with reagent, with about 5 drops of 

water using eye dropper; do not over wet.

$Press moistened pad firmly against test surface for about 2 min.

$Observe color of the pad or test surface for characteristic color.

$If no color occurs within 2 min, immediately check reagent using test 

verification card (press pad on card for 15 s).

STK4 Rhodizonate Pink or red $Remove all dust and dirt.

$Cut a notch at a diagonal down to substrate surface.

$Activate test swab by crushing on marked points, then shake and squeeze 

with tip facing down until yellow liquid comes to tip; use activated swab 

immediately.

$While squeezing swab, rub it on the notched area for 30 s.

$Complete test within 2 min.

$Observe swab and/or paint surface for characteristic color.

$If no characteristic color, use test confirmation card to check reagent.

$If the 2-min result is negative and lead chromate is suspected because of 

the paint color (e.g., yellow or green), place used swab in a plastic bag; 

check swab after 30 min and 60 min or next morning; alternatively, rub a 

crushed paint chip with a freshly activated swab, and check swab tip and 

crushed chip for up to 18 h. 

[Note: the experimental design for the study included obtaining data for 

STK4 response when determined after the three time periods: 

 - STK4a was the designation for the data set obtained when the first 

swab was examined within 2 min. 

 - STK4b was the designation for the data set obtained when the first 

used swab was later re-examined. 

 - STK4c was the designation for the data set obtained when the 

second swab in contact with chips was examined.

In the case of both STK4b and STK4c for test protocol uniformity, the 

swab examinations were made after the swabs were kept overnight.]

$For paint on plaster, because sulfates can interfere, if test appears 

negative, rub swab on confirmation card to check reactivity; if no color 

on test dot, test is not valid.

 

aThe information in this column was taken from kit manufacturers= instructions.
7

Table 2. Spot test kits and methods of use (cont.)

Spot 

Test

Kit 

Spot Test

Type

Characteristic

Color for Leada General Method of Usea

STK5 Rhodizonate Pink to rose/red $Clean surface with a lead-free paper towel, cloth, or wipe; allow to dry.

$Cut a V-notch in paint film to bare substrate.

$Place 2 drops of leaching solution on the tip of an unused swab.

$Rub swab tip gently on notch at 90E angle for 15 s.

$Rub swab tip on test card (i.e., reagent card) at 90E angle; before using 

the card for the first time, perform a QC test to assure reactivity.

$Observe test card and/or swab tip for characteristic color.

$For paint on plaster, separate paint from plaster (no specifics given) 

before lead test; if negative, perform a QC test immediately on swab.

STK6 Rhodizonate Pink to rose/red $Using borer (supplied with kit), cut through paint to substrate; do not 

collect any paint that prior to removal of borer.

$Remove borer from surface; collect chips on collection paper.

$Scrape any chips in cut onto collection paper.

$Using a stirring rod, dislodge chips in borer onto collection paper.

$Place paint in vial; grind for about 10 s using stirring rod.

$Add 3 drops of leaching solution into vial.

$Vigorously grind paint in vial for 10 s; then let stand for 20 s.

$Touch swab tip on leaching solution surface.

$Rub swab tip on test card (i.e., reagent card) at 90E angle; before using 

the card for the first time, perform a QC test to assure reactivity.

$Observe test card and/or swab tip for characteristic color. 

$For paint on plaster, eliminate it from paint specimen (no specifics given) 

before placing specimen in vial; if negative, immediately perform a QC 

test. 

STK7 Sulfide Light gray to 

dark gray to 

black

$Mix water and solid sodium sulfide to prepare reagent solution.

$Reagent may be applied to either painted surfaces or paint chips.

$For surfaces, make a diagonal cut (i.e., notch) through all paint layers.

$For chips, test both surfaces; cleave chips to test sandwiched layers.

$Apply reagent (a few drops) to the chips or painted surfaces, wait up to a 

couple of minutes for the characteristic color to form.

$Kit is not for use on painted metallic surfaces; chips are to be used.

[Note: the experimental design included tests on both notches and chips. 

The test of the notch was first conducted; if it was negative, then a test of 

a chip was conducted:

 - STK7a was the designation for the data set for notch tests

 - STK7b was the designation for the data set for chip tests.]

STK8 Sulfide Gray to black $Reagent may be applied to either painted surfaces or paint chips.

$For chips, include all layers down to the substrate; cut a cross-section and 

apply reagent to both surfaces and the cross-section.

$For painted surfaces, clean them with a non-abrasive solution, then rinse 

and dry.

$Notch surface exposing all layers of paint; add a drop of reagent on 

notch.

$After applying reagent, check for the characteristic color for up to 30 s.

$Kit is not for use on paint on metal substrates; remove a chip and test it.

[Note: the experimental design included tests on both notches and chips. 

The test of the notch was first conducted; if it was negative, then a test of 

a chip was conducted:

 - STK8a was the designation for the data set for notch tests

 - STK8b was the designation for the data set for chip tests.]

 

aThe information in this column was taken from kit manufacturers= instructions.
8

2.2 Fractional Factorial Design

A naive approach to experimentation would vary each of the seven factors individually, leaving all 

but one factor set at >typical' values for each experiment. This form of experimentation is highly 

inefficient, since it provides no information on potential interactions among the factors. A statistical 

design that requires testing at all combinations of levels for all factors is called a (full) factorial 

design. In cases of multi-factor studies, a full factorial design is seldom chosen for initial 

investigations because such a design would not be an efficient use of resources. Instead, a carefully 

chosen fraction of all possible combinations is usually selected to examine those effects considered 

most important. Such a design, called a fractional factorial, was used in this study for those spot test 

kits (STK1 & STK3-STK8) used on paint-film samples adhered to substrates (i.e., panels). 

In the case of paint-film samples adhered to substrates, all combinations of test kits, lead levels, 

operators, and lead pigment types were included in the design with the exception that Operator 4 and 

Operator 5 did not conduct tests with STK8 (Table 3). These four factors were considered to be the 

most important of the seven that might affect spot test response. Also included in this experimental 

design were four of the eight possible combinations of substrate, overlayer type, and overlayer 

thickness (Table 4). A benefit of this half-fractionation was that the design would reduce to a full 

factorial if test kit response was found not to be affected by any one of the three factors, substrate, 

overlayer type, and overlayer thickness. Tables 5A and 5B present a description of the 84 test 

combinations for the series of white lead and lead chromate samples, respectively, having paint films 

Table 3. Numbers of test kits, lead levels, operators, and lead pigment types selected in

 the experimental design for kits used to test paint films adhered to substrates

Level Selected for the Experimental Design

 Factor Op1 - Op3 Op4 & Op5

 Test Kit 7 6

 Lead Level 10 10

 Operator 3 2

 Lead Pigment Type 2 2

 

Table 4. Combinations of substrate, overlayer type, and overlayer thickness selected in

 the experimental design for kits used to test paint films adhered to substrates

Substratea Overlayer Type Overlayer Thickness

Reactive Latex Thin

Reactive Oil Thick

Non-reactive Oil Thin

Non-reactive Latex Thick

 

a

For sulfide-based and rhodizonate-based kits, the reactive substrates were steel and 
9

 plaster panels, respectively. For both kit types, the non-reactive substrate was wood.
10

 Table 5A. Experimental design combinations for white lead including four controls

Targeted Lead

Level, mg/cm2

Lead Pigment

Type Substratea

Overlayer

Type

Overlayer

Thickness

0 Not Applicable Non-reactive Latex Thick

0 Not Applicable Non-reactive Latex Thick

0 Not Applicable Non-reactive Latex Thick

0 Not Applicable Non-reactive Latex Thick

0 Not Applicable Non-reactive Latex Thick

0 Not Applicable Non-reactive Oil Thin

0 Not Applicable Reactive Latex Thin

0 Not Applicable Reactive Oil Thick

0.1 White Non-reactive Latex Thick

0.1 White Non-reactive Oil Thin

0.1 White Reactive Latex Thin

0.1 White Reactive Oil Thick

0.2 White Non-reactive Latex Thick

0.2 White Non-reactive Oil Thin

0.2 White Reactive Latex Thin

0.2 White Reactive Oil Thick

0.3 White Non-reactive Latex Thick

0.3 White Non-reactive Oil Thin

0.3 White Reactive Latex Thin

0.3 White Reactive Oil Thick

0.4 White Non-reactive Latex Thick

0.4 White Non-reactive Oil Thin

0.4 White Reactive Latex Thin

0.4 White Reactive Oil Thick

0.5 White Non-reactive Latex Thick

0.5 White Non-reactive Oil Thin

0.5 White Reactive Latex Thin

0.5 White Reactive Oil Thick

0.7 White Non-reactive Latex Thick

0.7 White Non-reactive Oil Thin

0.7 White Reactive Latex Thin

0.7 White Reactive Oil Thick

1.0 White Non-reactive Latex Thick

1.0 White Non-reactive Oil Thin

1.0 White Reactive Latex Thin

1.0 White Reactive Oil Thick

1.6 White Non-reactive Latex Thick

1.6 White Non-reactive Oil Thin

1.6 White Reactive Latex Thin

1.6 White Reactive Oil Thick

3.5 White Non-reactive Latex Thick

3.5 White Non-reactive Oil Thin

3.5 White Reactive Latex Thin

3.5 White Reactive Oil Thick

 

a

For sulfide-based and rhodizonate-based kits, the reactive substrates were steel and plaster panels,
11

 respectively. For both kit types, the non-reactive substrate was wood. 
12

 Table 5B. Experimental design combinations for lead chromate

Targeted Lead

Level, mg/cm2

Lead Pigment

Type Substratea

Overlayer

Type

Overlayer

Thickness

0 Not Applicable Non-reactive Latex Thick

0 Not Applicable Non-reactive Oil Thin

0 Not Applicable Reactive Latex Thin

0 Not Applicable Reactive Oil Thick

0.5 Lead Chromate Non-reactive Latex Thick

0.5 Lead Chromate Non-reactive Oil Thin

0.5 Lead Chromate Reactive Latex Thin

0.5 Lead Chromate Reactive Oil Thick

0.7 Lead Chromate Non-reactive Latex Thick

0.7 Lead Chromate Non-reactive Oil Thin

0.7 Lead Chromate Reactive Latex Thin

0.7 Lead Chromate Reactive Oil Thick

1.0 Lead Chromate Non-reactive Latex Thick

1.0 Lead Chromate Non-reactive Oil Thin

1.0 Lead Chromate Reactive Latex Thin

1.0 Lead Chromate Reactive Oil Thick

1.2 Lead Chromate Non-reactive Latex Thick

1.2 Lead Chromate Non-reactive Oil Thin

1.2 Lead Chromate Reactive Latex Thin

1.2 Lead Chromate Reactive Oil Thick

1.6 Lead Chromate Non-reactive Latex Thick

1.6 Lead Chromate Non-reactive Oil Thin

1.6 Lead Chromate Reactive Latex Thin

1.6 Lead Chromate Reactive Oil Thick

1.8 Lead Chromate Non-reactive Latex Thick

1.8 Lead Chromate Non-reactive Oil Thin

1.8 Lead Chromate Reactive Latex Thin

1.8 Lead Chromate Reactive Oil Thick

2.0 Lead Chromate Non-reactive Latex Thick

2.0 Lead Chromate Non-reactive Oil Thin

2.0 Lead Chromate Reactive Latex Thin

2.0 Lead Chromate Reactive Oil Thick

2.5 Lead Chromate Non-reactive Latex Thick

2.5 Lead Chromate Non-reactive Oil Thin

2.5 Lead Chromate Reactive Latex Thin

2.5 Lead Chromate Reactive Oil Thick

3.5 Lead Chromate Non-reactive Latex Thick

3.5 Lead Chromate Non-reactive Oil Thin

3.5 Lead Chromate Reactive Latex Thin

3.5 Lead Chromate Reactive Oil Thick

 

a

For sulfide-based and rhodizonate-based its, the reactive substrates were steel and plaster panels,

 respectively. For other kit types, the nonreactive substrate was wood.
13

adhered to substrates. Note in Table 5A that four additional samples having zero lead level, 

fabricated on a nonreactive substrate having a thick overlayer of latex paint, were included in the test 

series to increase the number of samples without lead. The final design for paint-film samples 

adhered to substrates required 2772 tests.

In the case of STK2 (for chips), only Operators 1 through 3 conducted the tests. A full factorial 

design was selected for this spot test kit. This design resulted in 84 test combinations for the series 

of white lead and lead chromate chip samples including four additional samples having zero lead level 

and a thick overlayer of latex paint. The final design for chips required 252 tests.

3. SPOT TEST KITS IN THE STUDY

Seven of the eight spot test kits were commercial products purchased directly from the kit 

manufacturers. The eighth kit, STK8, was obtained from a state laboratory that supplies sulfide kits 

to in-state professional lead inspectors and risk assessors. Three criteria were considered in the 

selection of test kits. First, the kits selected should be available to certified lead inspectors or risk 

assessors. Second, the kits selected should be representative of different protocols by which the spot 

testing is conducted in practice. For example, if two kits available from different manufacturers were 

based on the same reagent and used to test for the presence of lead in paint in the same, or essentially 

the same manner, then only one of the two would be selected. Third, the kits selected should be 

used to test for the presence of lead within the entire paint film. This criterion excluded kits intended 

to detect lead on the top surface of a paint film.

In ordering spot test kits from a manufacturer, it was requested that all of the kits be from the same 

production lot. Compliance with this request could not always be verified since lot number 

designations were not always indicated on the test kit. Instead, it was assumed that all of the test kits 

came from the same production lot if they were all received from a manufacturer in the same 

shipment. With the exception of STK6, all of the test kits from a given manufacturer were received 

in one shipment. In the case of STK6, when the test program was underway, quality control steps 

incorporated in the test procedure for this kit revealed that the rhodizonate reagent on some test 

cards had become inactive. The manufacturer replaced the unusable cards. Consequently, in all 

likelihood, not all of the test cards for STK6 were from the same production lot. An important 

lesson to be learned from this experience is that users must follow manufacturers= quality control 

steps (as well as other instructions) when testing for lead in paint with spot test kits.

Regarding the second selection criterion, the rhodizonate kits, STK3 through STK6, employ slightly 

different protocols (Table 2). Thus, all four of these test kits were selected for inclusion in the study. 

On the other hand, the protocols for sulfide kits that detect lead using a procedure that includes 

cutting a notch in the paint film were comparable. For this reason, it was intended to use STK1 as a 

representative sulfide kit. Preliminary testing using this kit indicated that the reagent would turn 

brown-to-black within 2 min after being placed on a lead-containing paint. However, the reagent 

also turned brown-to-black after it was placed on a non-lead-containing paint and on a glass 

microscope slide for 5 min to 10 min. 

It was beyond the scope of the study to examine reasons why STK1 produced brown-to-black color 

changes on nonleaded surfaces within 5 min to 10 min. Nevertheless, such observations raised 

serious questions regarding the assumption that STK1 was typical of other sulfide spot test kits. As 

the test program was underway, STK7 and STK8 kits were obtained. Analyses of the sodium sulfide 
14

concentrations of STK1, STK2, STK7, and STK8 reagent solutions were performed.* The results 

(Table 6) indicate a marked difference in sodium sulfide concentration between STK1 and the other 

three sulfide spot test kits. Moreover, the 0.51 % mean sodium sulfide concentration of STK1 was 

considerably less than the 6 % to 8 % range required of sulfide test kits used within the lead-paint 

inspection program conducted in the Commonwealth of Massachusetts [27]. Also, STK7 and STK8 

did not produce a brown-to-black color when these reagents were placed on the surfaces of 

nonleaded paint films. These observations and the results of the sodium sulfide analyses implied that 

STK1 was not typical and, thus, STK7 and STK8 kits were added to the test program.

 Table 6. Results of sodium sulfide analyses of sulfide-based spot test kits

 Sodium Sulfide Concentrationa, % CoVc

 

Spot Test Kit

Number of 

Samples min max mean sdb %

STK1 6 0.44 0.68 0.51 0.095 18

STK2 6 7.41 7.63 7.50 0.094 1.3

STK7 6 7.48 7.82 7.65 0.13 1.7

STK8 6 6.71 7.00 6.88 0.099 1.4

 

amass (g) of sodium sulfide dissolved in 100 mL water.

 

b

sd is the standard deviation from the mean.

 

cCoV is the coefficient of variation; CoV = [(sd/mean) x 100].

3.1 Kits Having Multiple Test Procedures.

From Table 2, the STK4 test procedure directs that the reagent-soaked (i.e., activated) swab be 

rubbed into the notch through the thickness of the paint film and that the kit response be determined 

within 2 min. It further requires that, if the 2-min response is negative and lead chromate is 

suspected in the paint, the used swab be kept for as long as overnight and re-examined for the 

characteristic color. Alternatively, a second activated swab is to be placed in contact with a crushed 

paint chip, kept for as long as 18 h, and examined for the characteristic color. Thus, the efficacy of 

STK4 when the response was determined according to these prescribed steps in the test protocol was 

tested in the experimental design: 

 $ STK4a was the designation for the data set obtained when the first swab was examined within 

2 min, 

 $ STK4b was the designation for the data set obtained whenever the first used swab was 

re-examined after setting overnight (i.e., a minimum of 16 h), and

 $ STK4c was the designation for the data set obtained whenever a second activated swab was 

examined after extended (i.e., overnight) contact with crushed paint chips.

 

*

The analysis was performed by the Environmental Lead Laboratory, State Laboratory Institute, 

Massachusetts Department of Public Health.
15

In this laboratory study, the STK4b and STK4c swab examinations were made after the swabs were 

kept overnight.

Also, from Table 2, the sulfide test kits STK7 and STK8 permit the application of the reagent either 

to a notch through the thickness of the paint film or onto a paint chip. Testing of the efficacy of 

STK7 and STK8 when used with notched surfaces and with chips was provided for in the 

experimental design. The test of the notch was first conducted; if it was negative, then a test of a 

chip was conducted:

 $ STK7a and STK8a were the designations for data sets from notch tests, and

 $ STK7b and STK8b were the designations for data sets from chip tests.

It is to be noted that, for the STK7 and STK8 kits, the manufacturers= recommended practice is not 

to apply the reagent to painted metal surfaces. For completeness, this prohibition was not followed 

in this study.

4. EXPERIMENTAL

4.1 Sample Preparation

4.1.1 Leaded Paint. Although the experimental design required 10 lead levels for each lead pigment 

type, paint films having 18 and 19 lead levels for white lead and lead chromate, respectively, were 

produced during the sample preparation phase of the study (Table 7). The additional lead levels 

were taken because, until preliminary spot test results were analyzed, the appropriate lead levels to 

be used in the test program were not known. 

 Table 7. Lead levels prepared in the study

 Lead Pigment Type 

 White Lead Lead Chromate 

Lead Level, 

mg/cm2

Level Included 

in Main Study?

Lead Level, 

mg/cm2

Level Included 

in Main Study?

0 yes 0 yes

0.05 ---

0.1 yes 0.1

0.2 yes 0.2

0.3 yes 0.3

0.4 yes 0.4

0.5 yes 0.5 yes

0.6 0.6

0.7 yes 0.7 yes

0.8 0.8

0.9 0.9

1.0 yes 1.0 yes

1.1 1.1

1.2 1.2 yes

1.4 1.4

1.6 yes 1.6 yes

1.8 1.8 yes

--- 2.0 yes

--- 2.5 yes
16

3.5 yes 3.5 yes
17

For both white lead and lead chromate samples, a paste was made by mixing the pigment into linseed 

oil and a small amount of mineral spirits. Then, each paste was well mixed with a commercial 

household alkyd paint to obtain Astock solutions@ that would provide paint films having a lead level 

of approximately 3.5 mg/cm2. Paint samples having lower targeted lead levels were made by diluting 

the stock solutions with the commercial household alkyd paint. The alkyd paint used for the 

dilutions was tinted beige for white lead and yellow for lead chromate to ensure that the leaded-paint 

films in all specimens for each type of lead had the same color and that different lead levels could not 

be distinguished visually.

4.1.2 Leaded-Paint Films. The leaded-paint films were prepared by spreading the pre-mixed paint 

(Section 4.1.1) on a smooth, nonporous surface using a drawdown blade. The drawdown technique 

was used to provide films having uniform thickness and width and length dimensions of 

approximately 200 mm by 450 mm (8 in by 18 in), which was larger than the area of the final test 

panel (Section 4.1.3). The dry film thickness of a Adraw@ was approximately 75 μm (0.003 in). 

Although the type of substrate included in the study was categorized, for experimental design 

purposes, as either nonreactive or reactive (i.e., causing interference) to the test kit reagent, three 

substrates were used in preparation of the test panels: wood, plaster, and steel. Wood was 

considered to be nonreactive for both rhodizonate and sulfide test kits. Plaster substrates, however, 

may interfere with the development of the characteristic color for rhodizonate test kits and result in 

false negative responses [6]. Metals present in steel substrates, in contrast, may react with sulfide 

test kits resulting in false positives [6]. For these reasons, the experiments were designed so that 

rhodizonate reagents were applied to specimens having plaster and wood substrates, and sulfide 

reagents were applied to specimens with steel and wood substrates.

For steel substrates, the leaded-paint films were drawn directly on 0.90 mm (0.036 in) thick, 

commercial panels sold for paint tests. However, wood and plaster are either not smooth or are too 

porous to create a uniformly thick, defect-free drawn film. Consequently, for these two substrates, a 

0.075 mm (0.003 in) thick mylar film and a 0.13 mm (0.005 in) thick release paper, respectively, 

were used in producing the drawn leaded-paint films. The mylar film remained in place when the 

leaded-paint films were adhered to the wood substrate; whereas the release paper was removed 

before adhering the leaded-paint films to the plaster substrate (Section 4.1.3). Regardless of 

substrate, wet films were stored overnight in a closed cabinet with forced air circulation at room 

temperature, 23 EC " 2 EC (73 EF " 4 EF), and then placed in a forced-air oven at about 75 EC 

(167 EF) for about 24 h.

The uniformity of the thickness of each draw was determined using a Series 6000 Coatings 

Thickness Gage (available from DeFelsko Co., Ogdensburg, NY).* Its calibration was checked 

against NIST-traceable coating thickness standards supplied by the gage manufacturer. This gage 

measures the thickness of nonmagnetic films on ferrous metal. Thus, when making the thickness 

measurements, the following steps were included:

 $ For draws on steel panels, the thickness measurements were made directly on the panels.

 

*Certain trade names or company products are mentioned in the text to specify adequately the experimental 

procedure and equipment used. In no case does such identification imply recommendation or endorsement by the 

National Institute of Standards and Technology, nor does it imply that the equipment is the best available for the 

purpose.
18

 $ For draws on mylar, the leaded-paint-coated sheet was set on a steel panel.

 $ For draws on release paper, the leaded-paint film was removed from the release paper and set on 

a steel panel. The thickness of the release paper was too variable to allow accurate thickness 

measurements.

To determine uniformity, a series of 24 thickness measurements was performed on each draw in the 

area of the film that was to be used to prepare final test panels. In all cases, the coefficient of 

variation (CoV) for the 24 measurements did not exceed 9 %, and only in three cases was it greater 

than 7 %. This was considered acceptable for the spot test panels.

4.1.3 Coated Test Panels. The dimensions of the leaded-paint film on wood, plaster, and steel test 

panels were approximately 150 mm by 175 mm (6 in by 7 in)*, with the width dimension parallel to 

the direction of the film draw. In the case of steel, two test panels were cut with a metal shear 

directly from the larger steel panels on which the draws were made. In the case of wood panels, two 

leaded-paint film sections having these dimensions were cut from the larger draws that had been 

prepared on mylar film. Then, in turn, the mylar on the (back) surface of the leaded-paint film 

section was manually adhered to a piece of nominal 13 mm (2 in) smooth-surfaced plywood using a 

contact-type spray adhesive (Elmer=s Spray Adhesive). The presence of the mylar film on the wood 

substrate did not affect the experimental results since the mylar is nonreactive to the spot test kit 

reagents and the paint films were notched through the mylar to the wood substrate. For plaster, two 

leaded-paint film sections having the above dimensions were cut from the larger draws that had been 

prepared on release paper. These lead-paint film sections (from which the release paper was 

removed before the thickness measurements were made) were manually adhered using the spray 

adhesive to pieces of nominal 13 mm (2 in) drywall that had been precoated with a 3 mm (c in) 

layer of gypsum plaster. The dimensions of the wood and plaster-coated drywall pieces were 

approximately 200 mm by 250 mm (8 in by 10 in). It is noted that the spray adhesive was 

nonreactive to the rhodizonate and sulfide reagents.

To complete fabrication of the test panels, the leaded-paint film/substrate assemblies were 

overcoated with thin and thick layers of latex and oil-based household paints. To aid adhesion of the 

latex paint to the leaded-paint films (which were heat-cured, alkyd based), a thin layer (about 

0.03 mm or 0.001 in) of an alkyd primer was applied by brush to the leaded-paint films. These 

primed panels then were cured overnight at ambient laboratory conditions prior to the application of 

the latex overlayers. Priming was not performed for the alkyd overlayer.

Each overlayer coat was applied with a roller. As a target value, it was assumed that each rollerapplied coat had a dry thickness of roughly 0.08 mm (0.003 in). The thin and thick panels were 

covered with two and generally 15 overlayer coats, respectively. Thus, the targeted overlayer 

thicknesses for the thin and thick panels were 0.16 mm and 1.2 mm (0.006 in and 0.048 in), 

respectively. After each roller application, the freshly-coated panel was placed in a forced-air oven 

at 75 EC (167 EF) for a minimum of 4 h. The thin panels were placed in the oven for an additional 

length of time such that they received about the same time of heat exposure as the thick panels. 

 

*

The ends of the draws were discarded when preparing films of this size.

Final overlayer thicknesses were estimated by making three thickness measurements on the steelsubstrate panels using the Series 6000 Coatings Thickness Gage. The measured thicknesses of the 

steel-substrate panels were considered to be typical of the wood- and plaster-substrate panels, 
19

because the overlayer application technique was the same for all three substrates. The results of 

these measurements on steel substrates showed that overcoat thicknesses of the thin panels ranged 

from 0.13 mm to 0.28 mm (0.005 in to 0.011 in) with a mean of 0.23 mm (0.009 in); whereas those 

of the thick panels ranged from 0.75 mm to 1.4 mm (0.030 in to 0.055 in) with a mean of 1 mm 

(0.4 in). 

4.1.4 Surface Test Grid and Panel Identification. An indelible grid was marked on the surface of 

each overlayered test panel. The grid consisted of 6 rows and 7 columns of squares measuring about 

25 mm by 25 mm (1 in by 1 in). Each grid square (referred to as a Atest square@) pinpointed the 

location at which a spot test was to be conducted or the location at which a leaded-paint sample was 

removed for instrumental lead analysis. Each test panel was also labeled with a three-number code 

for identification and a separate three-letter code for verification that the correct panel was used in a 

given spot test (Section 4.2).

4.1.5 Leaded-Paint Chips. The 200 mm by 450 mm (8 in by 18 in) nonleaded and leaded draws 

(Section 4.1.2) to be used in preparing chip specimens were cut into four strips having dimensions of 

approximately 200 mm by 88 mm (8 in by 3.5 in).∗ The spot testing of chips was a full 2 x 2 factorial 

experiment involving overlayer type and overlayer thickness and, thus, the four strips were randomly 

assigned to each overlayer type/thickness combination (i.e., thin/latex, thin/oil, thick/latex, and 

thick/oil). Coating application with a roller and subsequent cure of the overlayers were performed 

similarly to the procedures given in Section 4.1.3, although at least four overlayer coats were applied 

to the thin samples.

The thickness of the overlayer was measured using a micrometer. For thin chips, the thickness range 

was 0.33 mm to 0.63 mm (0.013 in to 0.025 in) with a mean of 0.50 mm (0.020 in); whereas, for 

thick chips, it was 0.88 mm to 1.5 mm (0.035 in to 0.060 in) with a mean of 1.2 mm (0.047 in). 

Each overlayered strip was sealed within two self-sealable (i.e., zip-lock) plastic storage bags, and 

immersed in liquid nitrogen for about 10 s. The frozen strips were set on the laboratory bench and 

broken into chips by immediately striking the storage bags with a rubber-faced mallet. After 

warming to room temperature, the chips were transferred to another plastic storage bag and labeled 

with both a three-number code and a three-letter code.

4.1.6 Selection of Lead Levels. The 10 lead levels incorporated in the test program were chosen on 

the basis of a preliminary experiment conducted using the series of finished white lead and lead 

chromate test panels having the lead levels given in Table 7. This preliminary experiment was 

designed to estimate the lowest lead level at which a positive test kit response was obtained, and was 

performed using STK1, STK3, STK4, and STK6.

4.1.7 Laboratory Lead Analysis. The lead levels assigned to the test panels and chips selected for 

inclusion in the study were determined quantitatively by a commercial laboratory. This laboratory 

was accredited in the National Lead Laboratory Accreditation Program (NLLAP) [28] and the 

measurements were performed using inductively coupled plasma (ICP) spectrometry according to 

NLLAP protocols. With the exception of test panels having targeted lead levels of 0 mg/cm2, three 

paint-film samples were removed from three test squares and sent to the commercial laboratory. The 

selected test squares, which were taken from the top, middle, and bottom sections of the panels, 

were the same for all test panels to help ensure that different test panels could not be identified by the 

 

*The ends of the draws were discarded when preparing these strips.
20

operators. The sampled test squares were also covered with masking tape to reduce the possibility 

that an individual test panel could be readily identifiedcation. For the 0 mg/cm2 lead level,∗ only one 

test square was analyzed per panel. The intent was to reaffirm that these panels had lead levels as 

targeted and that no contamination had occurred during panel preparation. Although only one test 

square of the 0 mg/cm2 lead-level panels was sampled, three test squares were masked with tape. 

For panels having lead levels greater than 0 mg/cm2, means and coefficients of variation (CoV) of the 

three lead analyses were calculated. In general, where the CoV was 20 % or less, then analyses were 

considered acceptable. Where the CoV was greater than 20 % or where an analysis was suspect 

(e.g., cracking of a sample container during shipment), one, two, or three additional paint-film 

samples were taken from the panels and subjected to lead analyses. In these cases, the mean of all 

measurements was calculated. Tables 8A through 8D list both the targeted and mean lead levels of 

all panels. It is evident in these tables that the mean lead levels were close to the targeted values. 

The mean values were used in all analyses described in Section 5.

The lead analyses of chips were performed by making one measurement for each chip sample. For 

the 0 mg/cm2 lead level, the result of the single lead measurement was taken as the mean lead level 

(as was the case for panels). For the other lead levels, the mean value of the lead determinations of 

the four chip samples fabricated from a single draw∗∗ was calculated. In all cases, the CoV was less 

than 20%. Tables 9A and 9B present the targeted and mean lead levels for the chips.

4.2 Testing and Data Recording

Operators conducted the spot tests according to protocols written for each of the eight spot test kits. 

For each protocol, the basic steps for the spot test kit were taken from the manufacturer=s 

instructions. Additional procedures were included to ensure the quality of the measurements. For 

example, manufacturers= instructions were generally not specific regarding steps to be taken in 

cleaning paint specimen surfaces and the cutting tools used to notch the paint-film panels. Also, 

manufacturers= instructions did not address formats for recording data. Because all testing was to be 

conducted at a single laboratory workstation, the cleaning procedures were important to avoid crosscontamination of the specimens.

Each protocol was reviewed by a manufacturer=s representative to assure that the steps given in the 

manufacturer=s instructions were being followed in the protocols. A magnifying glass 

(x5 magnification) attached to a flashlight was supplied to improve the operator=s ability to determine 

whether the substrate was exposed when a notch was cut in the paint-film panel, and to assist in 

judging whether the characteristic color change occurred in the notched area. In the case of sulfide 

test kits, operators were directed to cut two notches in the paint film (which was a step incorporated 

in this laboratory study complementary to the manufacturer=s instructions). One notch was to be 

 

*A lead level of 0 mg/cm2

 is the designation assigned to test panels for which lead was not added to the paint 

films. Measurements showed that the lead levels of these panels was < 0.009 mg/cm2. It is noted that a distinction 

between white lead and lead chromate for specimens having a 0 mg/cm2 lead level is artificial because such specimens 

do not contain lead. Nevertheless, the distinction is maintained in the discussions because of the balance of the 

experimental design.

**Recall from Section 4.1.5 that, in fabricating the chip samples, a single leaded-film draw was sectioned into 

four equally-sized strips. Then, because the chip spot tests were planned as a full factorial experiment, each strip was 

overcoated with one of the overlayer type/thickness combinations (i.e., thin/latex, thin/oil, thick/latex, and thick/oil).
21

treated with sulfide reagent; whereas the second notch was to be treated with water. This procedure 

allowed an operator to compare whether a Adarkening@ of the paint film in the treated notch was 

possibly due to a Awetting effect@ and not to the gray, black, or brown-to-black characteristic color 

change typical of sulfide test kits.

 Table 8A. Targeted and mean lead levels for the test panels having white lead

 and non-reactive substratesa

 Panel Description 

 

 Lead Level 

Test 

Panel 

ID Code Substrate

Type

Overlayer 

Type

Overlayer 

Thickness

Targeted

mg/cm2

 Mean 

mg/cm2

No. of

Samples

 CoV 

%

156 Wood Latex Thick 0 <0.009 1 Na

220 Wood Latex Thick 0 <0.009 1 Na

255 Wood Latex Thick 0 <0.009 1 Na

293 Wood Latex Thick 0 <0.009 1 Na

247 Wood Latex Thick 0 <0.009 1 Na

257 Wood Oil Thin 0 <0.009 1 Na

290 Wood Latex Thick 0.1 0.093 3 7.6

242 Wood Oil Thin 0.1 0.11 3 20.0

104 Wood Latex Thick 0.2 0.19 3 3.4

325 Wood Oil Thin 0.2 0.14 4 24.7

186 Wood Latex Thick 0.3 0.26 3 3.2

153 Wood Oil Thin 0.3 0.23 3 7.4

150 Wood Latex Thick 0.4 0.36 3 5.3

312 Wood Oil Thin 0.4 0.31 4 26.1

208 Wood Latex Thick 0.5 0.46 4b1.6

134 Wood Oil Thin 0.5 0.49 4 17.6

240 Wood Latex Thick 0.7 0.63 3 8.0

228 Wood Oil Thin 0.7 0.62 3 12.8

267 Wood Latex Thick 1.0 0.92 3 4.0

128 Wood Oil Thin 1.0 0.92 4 16.0

244 Wood Latex Thick 1.6 1.34 4 6.6

200 Wood Oil Thin 1.6 1.33 3 3.2

217 Wood Latex Thick 3.5 3.15 3 1.6

172 Wood Oil Thin 3.5 3.75 3 5.3

 

aThe first four rows represent additional zero-lead samples added to the factorial design.

 

b

For this panel, the commercial laboratory reported damage to a sample container during shipment

 of the initial samples sent for lead analyses. Consequently, an additional sample was analyzed. 

 The results of the four measurements are reported.
22

 Table 8B. Targeted and mean lead levels for the test panels having white lead

 and reactive substrates

 Panel Description Lead Level 

Test

Panel 

ID Code

Substrate

Type

Overlayer 

Type

Overlayer 

Thickness

Targeted

mg/cm2

 Mean 

mg/cm2

No. of

Samples

 CoV 

%

107 Plaster Latex Thin 0 <0.009 1 Na

313 Plaster Oil Thick 0 <0.009 1 Na

322 Steel Latex Thin 0 <0.009 1 Na

101 Steel Oil Thick 0 <0.009 1 Na

188 Plaster Latex Thin 0.1 0.10 3 2.6

285 Plaster Oil Thick 0.1 0.094 3 1.3

221 Steel Latex Thin 0.1 0.09 3 2.6

168 Steel Oil Thick 0.1 0.087 3 3.4

234 Plaster Latex Thin 0.2 0.19 3 4.1

237 Plaster Oil Thick 0.2 0.19 3 5.7

269 Steel Latex Thin 0.2 0.17 3 5.2

180 Steel Oil Thick 0.2 0.18 3 6.1

165 Plaster Latex Thin 0.3 0.28 3 0.8

138 Plaster Oil Thick 0.3 0.31 3 2.0

203 Steel Latex Thin 0.3 0.23 4 18.0

135 Steel Oil Thick 0.3 0.23 3 6.1

262 Plaster Latex Thin 0.4 0.39 3 1.7

129 Plaster Oil Thick 0.4 0.43 3 4.0

222 Steel Latex Thin 0.4 0.32 3 8.5

189 Steel Oil Thick 0.4 0.29 3 5.0

206 Plaster Latex Thin 0.5 0.48 3 4.4

281 Plaster Oil Thick 0.5 0.50 3 2.6

326 Steel Latex Thin 0.5 0.44 3 7.7

130 Steel Oil Thick 0.5 0.36 3 5.9

179 Plaster Latex Thin 0.7 0.70 3 6.6

292 Plaster Oil Thick 0.7 0.66 3 1.5

224 Steel Latex Thin 0.7 0.58 3 6.3

226 Steel Oil Thick 0.7 0.54 3 10.0

256 Plaster Latex Thin 1.0 1.04 3 8.0

102 Plaster Oil Thick 1.0 0.96 3 4.5

278 Steel Latex Thin 1.0 0.78 4 7.3

246 Steel Oil Thick 1.0 0.86 3 6.7

251 Plaster Latex Thin 1.6 1.53 3 8.3

279 Plaster Oil Thick 1.6 1.54 3 0.5

284 Steel Latex Thin 1.6 1.39 3 5.2

123 Steel Oil Thick 1.6 1.46 3 5.0

177 Plaster Latex Thin 3.5 3.53 3 13.4

127 Plaster Oil Thick 3.5 3.88 4 6.4

215 Steel Latex Thin 3.5 3.20 3 4.7

145 Steel Oil Thick 3.5 3.39 3 2.4
23

 Table 8C. Targeted and mean lead levels for the test panels having lead

chromate and non-reactive substrates

 Panel Description Lead Level 

Test

Panel 

ID Code

Substrate

Type

Overlayer 

Type

Overlayer 

Thickness

Targeted

mg/cm2

 Mean 

mg/cm2

No. of

Samples

 CoV 

%

277 Wood Latex Thick 0 <0.009 1 Na

316 Wood Oil Thin 0 <0.009 1 Na

296 Wood Latex Thick 0.5 0.48 3 7.4

143 Wood Oil Thin 0.5 0.42 4 29.3

258 Wood Latex Thick 0.7 0.64 4 8.9

204 Wood Oil Thin 0.7 0.67 3 3.3

230 Wood Latex Thick 1.0 0.89 4 4.4

320 Wood Oil Thin 1.0 1.07 3 14.0

265 Wood Latex Thick 1.2 1.15 6a4.8

146 Wood Oil Thin 1.2 1.08 5 17.4

264 Wood Latex Thick 1.6 1.39 5 15.9

291 Wood Oil Thin 1.6 1.40 3 14.0

176 Wood Latex Thick 1.8 1.69 6a5.9

207 Wood Oil Thin 1.8 1.52 4 23.0

239 Wood Latex Thick 2.0 1.85 3 7.3

213 Wood Oil Thin 2.0 1.65 3 16.7

328 Wood Latex Thick 2.5 2.33 6 16.8

332 Wood Oil Thin 2.5 2.06 3 2.9

122 Wood Latex Thick 3.5 3.35 3 24.0b

137 Wood Oil Thin 3.5 2.84 3 8.2

 

a

For this panel, the commercial laboratory reported damage to the sample containers during shipment

 of the initial samples sent for lead analyses. Consequently, three additional samples were analyzed. 

 The results of the six measurements are reported.

 

bAlthough the CoV for this panel was greater than 20 %, additional samples were not analyzed 

 because the variability of the three measurements was considered acceptable for the 3.5 mg/cm2 level.
24

 Table 8D. Targeted and mean lead levels for the test panels having lead

 chromate and reactive substrates

 Panel Description Lead Level 

Test

Panel 

ID Code

Substrate

Type

Overlayer 

Type

Overlayer 

Thickness

Targeted

mg/cm2

 Mean 

mg/cm2

No. of

Samples

 CoV 

%

175 Plaster Latex Thin 0 <0.009 1 na

202 Plaster Oil Thick 0 <0.009 1 na

303 Steel Latex Thin 0 <0.009 1 na

319 Steel Oil Thick 0 <0.009 1 na

250 Plaster Latex Thin 0.5 0.47 3 5.4

140 Plaster Oil Thick 0.5 0.51 3 0.9

223 Steel Latex Thin 0.5 0.49 3 7.8

308 Steel Oil Thick 0.5 0.43 3 7.7

274 Plaster Latex Thin 0.7 0.69 3 6.2

163 Plaster Oil Thick 0.7 0.73 4 3.7

289 Steel Latex Thin 0.7 0.57 3 6.7

310 Steel Oil Thick 0.7 0.52 3 10.8

323 Plaster Latex Thin 1.0 0.94 3 7.4

297 Plaster Oil Thick 1.0 0.94 3 3.2

216 Steel Latex Thin 1.0 0.74 3 12.5

302 Steel Oil Thick 1.0 0.75 3 14.4

126 Plaster Latex Thin 1.2 1.09 3 4.0

178 Plaster Oil Thick 1.2 1.20 3 0.7

231 Steel Latex Thin 1.2 0.98 3 5.7

113 Steel Oil Thick 1.2 0.92 3 6.0

148 Plaster Latex Thin 1.6 1.46 3 3.1

183 Plaster Oil Thick 1.6 1.44 3 7.1

158 Steel Latex Thin 1.6 1.34 3 11.0

124 Steel Oil Thick 1.6 1.48 3 6.5

249 Plaster Latex Thin 1.8 1.708 3 3.2

253 Plaster Oil Thick 1.8 1.62 3 6.1

187 Steel Latex Thin 1.8 1.55 3 8.1

315 Steel Oil Thick 1.8 1.57 3 4.7

139 Plaster Latex Thin 2.0 1.98 3 2.8

141 Plaster Oil Thick 2.0 1.86 4 5.6

263 Steel Latex Thin 2.0 1.56 3 7.3

294 Steel Oil Thick 2.0 1.89 3 1.9

333 Plaster Latex Thin 2.5 2.51 3 3.5

330 Plaster Oil Thick 2.5 2.30 3 9.5

329 Steel Latex Thin 2.5 1.72 3 8.0

331 Steel Oil Thick 2.5 2.25 3 14.0

167 Plaster Latex Thin 3.5 3.29 3 1.3

205 Plaster Oil Thick 3.5 3.64 4 2.6

199 Steel Latex Thin 3.5 3.09 3 11.3

142 Steel Oil Thick 3.5 2.81 3 4.7
25

 Table 9A. Targeted and mean lead levels for chips with white leada

 Sample Description Lead Level 

Test

Panel 

ID Code

Sample

Type

Overlayer 

Type

Overlayer 

Thickness

Targeted

mg/cm2

 Mean 

mg/cm2

No. of

Samples

 CoV 

%

C948 Chips Latex Thick 0 <0.009 1 na

C949 Chips Latex Thick 0 <0.009 1 na

C950 Chips Latex Thick 0 <0.009 1 na

C951 Chips Latex Thick 0 <0.009 1 na

C802 Chips Latex Thick 0 <0.009 1 na

C812 Chips Latex Thin 0 <0.009 1 na

C806 Chips Oil Thick 0 <0.009 1 na

C938 Chips Oil Thin 0 <0.009 1 na

C914 Chips Latex Thick 0.1 0.093 4 11.4

C878 Chips Latex Thin 0.1 0.093

C887 Chips Oil Thick 0.1 0.093

C935 Chips Oil Thin 0.1 0.093

C908 Chips Latex Thick 0.2 0.17 4 16.0

C809 Chips Latex Thin 0.2 0.17

C894 Chips Oil Thick 0.2 0.17

C876 Chips Oil Thin 0.2 0.17

C930 Chips Latex Thick 0.3 0.28 4 7.9

C848 Chips Latex Thin 0.3 0.28 

C869 Chips Oil Thick 0.3 0.28

C877 Chips Oil Thin 0.3 0.28

C842 Chips Latex Thick 0.4 0.38 4 7.8

C880 Chips Latex Thin 0.4 0.38

C918 Chips Oil Thick 0.4 0.38

C826 Chips Oil Thin 0.4 0.38

C920 Chips Latex Thick 0.5 0.46 4 11.1

C815 Chips Latex Thin 0.5 0.46

C845 Chips Oil Thick 0.5 0.46

C910 Chips Oil Thin 0.5 0.46

C844 Chips Latex Thick 0.7 0.61 4 11.8

C825 Chips Latex Thin 0.7 0.61

C832 Chips Oil Thick 0.7 0.61

C839 Chips Oil Thin 0.7 0.61

C801 Chips Latex Thick 1.0 0.95 4 10.6

C831 Chips Latex Thin 1.0 0.95

C931 Chips Oil Thick 1.0 0.95

C849 Chips Oil Thin 1.0 0.95

C817 Chips Latex Thick 1.6 1.45 4 4.0

C922 Chips Latex Thin 1.6 1.45

C871 Chips Oil Thick 1.6 1.45

C819 Chips Oil Thin 1.6 1.45

C873 Chips Latex Thick 3.5 3.51 4 10.4

C883 Chips Latex Thin 3.5 3.51

C847 Chips Oil Thick 3.5 3.51

C874 Chips Oil Thin 3.5 3.51

aThe first four rows represent additional zero-lead samples added to the factorial design.
26

 Table 9B. Targeted and mean lead levels for chips with lead chromate

 Sample Description Lead Level 

Test

Panel 

ID Code

Sample

Type

Overlayer 

Type

Overlayer 

Thickness

Targeted

mg/cm2

 Mean 

mg/cm2

No. of

Samples

 CoV 

%

C940 Chips Latex Thick 0 <0.006 1 na

C927 Chips Latex Thin 0 <0.006 1 na

C916 Chips Oil Thick 0 <0.006 1 na

C852 Chips Oil Thin 0 <0.006 1 na

C866 Chips Latex Thick 0.5 0.47 4 11.8

C864 Chips Latex Thin 0.5 0.47

C822 Chips Oil Thick 0.5 0.47

C851 Chips Oil Thin 0.5 0.47

C925 Chips Latex Thick 0.7 0.64 4 9.8

C889 Chips Latex Thin 0.7 0.64

C841 Chips Oil Thick 0.7 0.64

C830 Chips Oil Thin 0.7 0.64

C905 Chips Latex Thick 1.0 0.93 4 7.2

C943 Chips Latex Thin 1.0 0.93

C813 Chips Oil Thick 1.0 0.93

C855 Chips Oil Thin 1.0 0.93

C865 Chips Latex Thick 1.2 1.06 4 10.2

C861 Chips Latex Thin 1.2 1.06

C923 Chips Oil Thick 1.2 1.06

C881 Chips Oil Thin 1.2 1.06

C890 Chips Latex Thick 1.6 1.46 4 10.1

C854 Chips Latex Thin 1.6 1.46

C850 Chips Oil Thick 1.6 1.46

C928 Chips Oil Thin 1.6 1.46

C947 Chips Latex Thick 1.8 1.67 4 7.5

C828 Chips Latex Thin 1.8 1.67

C944 Chips Oil Thick 1.8 1.67

C898 Chips Oil Thin 1.8 1.67

C888 Chips Latex Thick 2.0 1.85 4 5.2

C821 Chips Latex Thin 2.0 1.85

C853 Chips Oil Thick 2.0 1.85

C804 Chips Oil Thin 2.0 1.85

C875 Chips Latex Thick 2.5 2.19 4 8.6

C934 Chips Latex Thin 2.5 2.19

C942 Chips Oil Thick 2.5 2.19

C816 Chips Oil Thin 2.5 2.19

C933 Chips Latex Thick 3.5 3.17 4 3.1

C932 Chips Latex Thin 3.5 3.17

C921 Chips Oil Thick 3.5 3.17

C811 Chips Oil Thin 3.5 3.17
27

NIST staff reviewed the intent of the study and the general use of spot test kits with each operator 

after initial arrival at the laboratory. The review also included a discussion of the effect of lead 

pigment solubility on spot test kit response. Operators were required to practice cutting notches in 

paint films on typical test specimens until acceptable notches could be made without difficulty. 

Before beginning a series of measurements using a given test kit, each operator was trained to follow 

the protocol for that kit. Training included testing panels (or chips) that had been prepared 

identically to the test panels (or chips) in the testing program. These training panels (or chips) 

contained either white lead or lead chromate at lead levels covering the range of values in the test 

program.

For each series of 84 tests with a given kit, the operator was provided with a data form for recording 

the test results (i.e., positive or negative) along with those of quality control tests conducted during 

the series. For tests that were positive, the operator also recorded the intensity of the characteristic 

color. For sulfide, the designated intensity levels were faint gray, light gray, dark gray, and black or 

brown; for rhodizonate, they were faint pink, pink, bright pink, and red. Analyses of the recorded 

color intensities indicated that they contributed little to the conclusions of the study. Thus, the color 

intensity data are not generally discussed in this report.

The 84 tests were performed according to a randomly-selected sequence, which was pre-recorded on 

the data form using the number codes marked on the test panels (Sections 4.1.4 and 4.1.5). Also 

pre-recorded on the data form was the randomly-selected test square on which the spot test of a 

given panel was to be performed. After completing a spot test, the operator wrote the sample letter 

code (Sections 4.1.4 and 4.1.5) on the data form. This written letter code was subsequently crosschecked with the pre-recorded number code to verify that the operator performed the test on the 

correct panel (or chips). No errors were detected.

5. RESULTS AND DISCUSSION

5.1 False Negatives and False Positives

One measure of the performance of a spot test kit is the percent of false negatives and false positives 

obtained in conducting a series of tests [8,19,20]. As previously indicated, a false negative is a test 

result that is negative for lead, but the true value is above the selected lead level [8]. Conversely, a 

false positive is a test result that is positive for lead, but the true value is below the selected lead 

level. In the present report, the selected lead levels for analyzing false positives and false negatives 

are 0 mg/cm2 and 1 mg/cm2 (i.e., the definition level of lead-based paint [2,3]). Because many of the 

spot test kits displayed marked differences in performance depending upon lead pigment type (see 

discussions that follow), the false negatives and false positives observed in the study are tabulated for 

both white lead and lead chromate.

Table 10A presents the false negatives and false positives for the 0 mg/cm2 lead level∗. The response 

of the kits is seen to vary considerably. For example, the percent of false negatives for white lead 

ranged from 2 % to 42 %; whereas that for lead chromate ranged from 1 % to 82 %. Thus, although 

some kits displayed considerably less false negatives than others, all erroneously indicated lead in 

some specimens when none was present. Similarly, although with the exception of STK2 the percent 

of false positives was low (# 13%) for both lead pigment types, the tabulation

 

*A false positive for the 0 mg/cm2

 lead level indicates that a positive result was obtained for a test panel to 

which no lead had been added and for which the measured lead level was at least < 0.009 mg/cm2.
28

 Table 10A. False negatives and false positivesa at a lead level of 0 mg/cm2

Lead

Pigment

Type Kit

Total

No. of

Observations

 False Negatives 

No. % 

Total

No. of

Observations

 False Positives 

No. % 

STK1 180 72 40 60 8 13

STK2 108 5 5 36 18 50

STK3 180 3 2 60 0 0

STK4a 180 14 8 60 2 3

STK4b 180 14 8 60 2 3

STK4c 180 14 8 60 3 5

STK5 180 64 36 60 4 7

STK6 180 49 27 60 2 3

STK7a 180 49 27 60 5 8

STK7b 180 21 12 60 7 12

STK8a 108 45 42 36 1 3

White

Lead

STK8b 108 19 18 36 2 6

STK1 180 96 53 60 8 13

STK2 108 45 42 36 18 50

STK3 180 112 62 60 0 0

STK4a 180 56 31 60 2 3

STK4b 180 46 26 60 2 3

STK4c 180 2 1 60 3 5

STK5 180 147 82 60 4 7

STK6 180 145 81 60 2 3

STK7a 180 12 7 60 5 8

STK7b 180 4 2 60 7 12

STK8a 108 12 11 36 1 3

Lead

Chromate

STK8b 108 3 3 36 2 6

a

A false negative has been defined as a test result that is negative for lead, but the true value is above

 the selected lead level [8]. Conversely, a false positive is a test result that is positive for lead, but the 

 true value is below the selected lead level. A false positive for the 0 mg/cm2 lead level indicates 

 that a positive result was obtained for a test panel to which no lead had been added and for which the

 measured lead level was at least < 0.009 mg/cm2.

(Table 10A) showed that many tests indicated lead in a specimen when none was present. The 

response of STK3 should be noted; this kit had no false positives at the 0 mg/cm2 lead level.

The finding that STK2 had 50 % false positives at the 0 mg/cm2

 lead level was attributed to the 

criterion against which the operators were instructed to judge whether a characteristic color change 

occurred. The STK2 manufacturer=s procedure (Table 2) included adding drops of sulfide reagent to 

vinegar in which paint chips were placed. In this study, the operators were trained to record a 
29

positive result when the vinegar changed from clear to a faint gray (or darker) color∗ upon addition 

of the sulfide reagent. In preliminary experiments in which sulfide reagent was added to vinegar 

without paint chips, it was observed that a milky white coloration was produced. The operators 

apparently had difficulty in distinguishing between the milky white and faint gray colors. In this 

regard, when the STK2 data sets were analyzed by taking the results as positive for only those tests 

for which the operators indicated that the characteristic color change was Adark gray@ or 

Abrown/black,@ then the number of false positives at the 0 mg/cm2 lead level was zero.

The 1.0 mg/cm2 lead level for false negatives and false positives was selected because this value 

represents the Federal level at which a paint is classified as being lead-based [2,3]. The question 

examined is: Can spot test kits in a controlled laboratory study distinguish between lead-based paint 

and nonlead-based paint? If spot test kits are able to provide such differentiation, then the percent of 

both false negatives and false positives at the 1.0 mg/cm2 lead level should be low. Note that past 

field studies [8,19] have not found this to be the case.

Table 10B summarizes the false negatives and false positives at the 1.0 mg/cm2 lead level for white

lead and lead chromate. Regarding false negatives, the results varied considerably depending upon 

the lead pigment type. For white lead, the percent of false negatives was generally low (# 4 %), 

except for STK1 and STK5. In five cases (STK2, STK3, STK6, STK7b, and STK8b), no false 

negatives were observed. In contrast, for lead chromate, only STK4c, STK7a, STK7b, STK8a, and 

STK8b had low percents (#5 %) of false negatives. Regarding false positives, the vast majority 

(about 85 %) of the spot tests for both lead pigment types showed percents greater than 30 %. That 

is, most spot test kits gave positive responses when the true value was less than 1.0 mg/cm2. These 

results were consistent with those from previously reported field studies [8,19].

The percents of false negatives at the 1.0 mg/cm2 lead level (Table 10B) provide qualitative evidence 

that, except for STK1 and STK5, the spot test kits may be useful as a negative screen for lead-based 

paint having white lead pigments, but generally not for paints having lead chromate pigments. A 

negative screen is a test for which a negative result indicates a low probability of lead $ 1 mg/cm2. 

Only STK4c, STK7a/b, and STK8a/b showed percents of false negatives less than 5 %, indicating 

possible use as negative screens for lead chromate. For the remaining kits, the false negatives ranged 

from 20 % to 80 %. Thus, many of the kits were not able to detect lead chromate consistently when 

it was present at levels $ 1.0 mg/cm2. Further discussion of negative screens is given in Section 5.4. 

The suggestion of using a spot test kit for screening purposes has been previously proposed [20,29]. 

For example, based on a field investigation that included detection of lead in paint using a specific 

rhodizonate spot test kit (STK4 in the present study), Ashley et al. [20] suggested that rhodizonate 

spot test kits have potential use as an in-situ screening technique. Although they did not specifically 

indicate limiting kit use to that of a negative screen for lead-based paint, such a limitation may be 

implied as they reported that their rhodizonate kit gave predominantly positive results for paint 

specimens having lead levels above about 0.25 mg/cm2. This finding from the Ashley et al. field 

investigation compared well with the results in the present laboratory study for STK4 tests 

performed on white lead specimens, which showed primarily positive response for lead levels above 

0.2 mg/cm2. It is noted that Ashley et al. [20] performed tests with the rhodizonate spot test kit 

according to the STK4b procedure in the present study.

 

*The manufacturer=s instructions defined the characteristic color as brown or black upon addition of the 

sulfide reagent (Table 2).
30

 Table 10B. False negatives and false positivesa at a lead level of 1.0 mg/cm2

Lead

Pigment

Type

Kit

Total

No. of

Observations

 False Negatives 

No. % 

Total

No. of

Observations

 False Positives 

No. % 

STK1 40 5 13 200 81 41

STK2 30 0 0 114 91 80

STK3 45 0 0 195 132 68

STK4a 45 1 2 195 124 64

STK4b 45 1 2 195 124 64

STK4c 45 1 2 195 125 64

STK5 45 12 27 195 87 45

STK6 45 0 0 195 88 45

STK7a 40 1 3 200 97 49

STK7b 40 0 0 200 126 63

STK8a 24 1 4 120 41 34

White

Lead

STK8b 24 0 0 120 67 56

STK1 115 57 50 125 34 27

STK2 69 29 42 75 41 55

STK3 125 75 60 115 18 16

STK4a 125 33 26 115 34 30

STK4b 125 25 20 115 36 31

STK4c 125 1 1 115 57 50

STK5 125 100 80 115 12 10

STK6 125 97 78 115 9 8

STK7a 115 6 5 125 64 51

STK7b 115 2 2 125 70 56

STK8a 69 3 4 75 31 41

Lead

Chromate

STK8b 69 1 1 75 39 52

a

A false negative has been defined as a test result that is negative for lead, but the true value is above

 the selected lead level [8]. Conversely, a false positive is a test result that is positive for lead, but the 

 true value is below the selected lead level.

Some readers may have interest in percents of false negatives and false positives for selected lead 

levels other than the two discussed herein. Appendix A provides tabulations of false negatives and 

false positives for lead levels of 0.5 mg/cm2 and 0.7 mg/cm2.

5.1.1 Effect of STK4 Test Procedure. As outlined in Table 2, the STK4 instructions stipulate 

additional testing when the STK4a procedure produces a negative result and the presence of lead 

chromate is suspected. In this case, both STK4b and STK4c procedures are then performed. During 

testing, the operators correctly distinguished the white lead specimens from the lead chromate 

specimens, which was attributed to the noticeable difference in color between the leaded-paint layers. 
31

Consequently, the operators performed STK4b and STK4c only for the lead chromate specimens, 

and the data sets for STK4a, STK4b and STK4c were identical for white lead specimens*. 

As can be seen from Table 10B, the STK4 procedure had a notable effect on the percent of false 

negatives and false positives for lead chromate specimens. In particular, only one false negative 

(1 %) was observed for STK4c; whereas the false negative percents were 26 % and 20 % for STK4a 

and STK4b, respectively. Apparently, allowing the paint chips to be in contact with the swab for 

long exposure times (e.g., overnight as done in the present study) markedly increased the sensitivity 

of STK4 to lead chromate. It might be expected that this additional sensitivity to lead would result 

in more false positive results for specimens which have no lead. In fact, this was true, but there was 

only 1 more false positive for STK4c than for STK4b (Table 10A). This is only a small penalty to 

pay for so much added sensitivity. It may be that the STK4c procedure should always be conducted 

when the STK4a procedure produces a negative response. 

5.2 Factors Affecting Spot Test Kit Response

5.2.1 Method of Analysis. To assess statistical significance, an often useful method is to calculate Fstatistics and their associated P-values [30]. This approach was taken in evaluating some factors 

designed into the study (i.e., overlayer type, overlayer thickness, lead pigment type, and operator) for 

their effects on spot test kit response. An F-statistic is a ratio of variance estimates for which the 

numerator is calculated under the assumption of no effect, and the denominator is calculated 

allowing for the effect. Under the assumption of no effect, an F-statistic has a probability distribution 

that does not depend on any unknown quantities. Tabulations of this distribution are available for 

comparison with F-statistics obtained from experimental data [31]. F-statistic ratios substantially 

exceeding 1 tend to indicate that an effect is not due to chance. A measure of how unlikely that an 

observed effect is due to chance is provided by the P-value. For purposes of the present report, the 

P-value is the probability of observing an F-statistic as large or larger than the one obtained, if the 

effect was not present. Conventionally, effects which have P-values less than 0.05 are referred to as 

being statistically significant.

5.2.2 Effect of Overlayer Type and Overlayer Thickness. Table 11 presents the F-statistics and Pvalues for the analysis of overlayer type and overlayer thickness. As evidenced in the table, STK6 

was the only kit that displayed a statistically significant overlayer effect. Note the large F-statistic 

and consequently small P-value. In fact, this P-value is sufficiently small that the overlayer 

type/overlayer thickness effect for STK6 is virtually certain to be real. Further analysis (not shown) 

of the STK6 data was performed to differentiate whether the effect in Table 11 was due to overlayer 

type or overlayer thickness. The result of this analysis indicated that the effect was either due to 

overlayer type or to the interaction of overlayer thickness with substrate. A decision between the 

two possibilities could not be made with certainty from the data alone because only half of the eight 

possible combinations of the three factors, overlayer type, overlayer thickness, and substrate, were 

included in the design. However, the main effect for thickness was found to be very small for STK6, 

suggesting that the overlayer thickness-substrate interaction is less likely than overlayer type to be 

causing the significant STK6 result in Table 11.

 

*

For statistical analysis purposes, it was necessary to repeat the results of the STK4a tests for white lead 

specimens in the STK4b and STK4c data sets.
32

 Table 11. F-tests of the significance of overlayer type and overlayer thickness

 Analysis Result 

Spot Test Kit F-Statistic P-Value

STK1 1.07 0.343

STK2 1.88 0.133

STK3 1.20 0.303

STK4a 0.84 0.434

STK4b 0.87 0.418

STK4c 2.40 0.092

STK5 0.15 0.862

STK6 9.94 < 0.001 

STK7a 0.53 0.599

STK7b 0.74 0.478

STK8a 0.13 0.883

STK8b 0.63 0.535

Consideration of the STK6 test procedure supported the hypothesis that the STK6 result in Table 11 

was due to overlayer type, and that it may have been an artifact of the experimentation. In 

conducting an STK6 test, chips are cut from the paint film, placed in a vial, and ground with a plastic 

stirring rod (Table 2). Observation of the grinding of lead chromate specimens found that yellow 

particles (i.e., the lead chromate layer) were readily visible in the vial when the overlayer was latex. 

In contrast, it was difficult to see the yellow particles when the overlayer was oil. Thus, when latex 

was the overlayer, the leaded paint was apparently more accessible to the STK6 leaching solution 

than when the overlayer was oil. Although this effect was not further investigated, it was 

hypothesized that the adhesion of the latex overlayer to the oil-based leaded-paint film was less than 

that of the oil overlayer to the oil-based leaded-paint film.

As a final comment, the lack of an overlayer type/overlayer thickness effect for the other spot test 

kits was, in great part, attributed to the control that was applied by the operators in conducting the 

tests. Consistent with manufacturers= instructions, operators were directed that all layers of paint 

were to be clearly exposed to the substrate when notches were made in, or chips were cut from, the 

paint films. This was to be verified using the magnifying lens attached to the flashlight, if necessary. 

Furthermore, reagents were to be applied to the exposed edges of the paint films, or chips as 

appropriate. Apparently, following these instructions allowed the spot test reagent to be placed 

consistently in contact with the leaded paint film regardless of whether the overlayer was latex or oil, 

and also thick or thin. Consequently, no overlayer type/overlayer thickness effect was found. This 

finding reinforces the importance of exposing all layers of paint to the spot test reagent, as given in 

manufacturers= kit instructions.

5.2.3 Effect of Lead Pigment Type. Table 12 presents the F-statistics and P-values for the analysis

of the effect of lead pigment type. The results indicate that a lead pigment type effect was present in 

all cases. STK4b had a P-value of 0.056, which is only slightly greater than the 0.05 significance 

level. Lead chromate was more difficult to detect with the spot test kits than white lead. These 
33

experimental results provide a measure of the extent to which the lack of solubility of lead chromate 

affects spot test kit response.

5.2.4 Effect of Operator. Table 13 presents the F-statistics and P-values for the analysis of operator 

effect. The analyses were conducted separately for white lead and lead chromate because the lead 

pigment effect was so strong (Section 5.2.3). For white lead, which was readily detected at low lead 

levels, a statistically significant operator effect was only seen for STK1, although the effect was only 

marginally not significant for STK5. Similarly, for lead chromate, an operator effect was also 

generally present for those kits that yielded high percents of false negatives at the 1 mg/cm2 lead 

level. In summary, for both white lead and lead chromate, operator effects tended to be more 

pronounced when relatively low kit sensitivity presented more of a challenge to the operator.

 Table 12. F-tests of the significance of lead pigment type

 Analysis Result 

Spot Test Kit F-Statistic P-Value

STK1 4.03 0.045

STK2 39.42 < 0.001 

STK3 118.91 < 0.001 

STK4a 9.04 0.003

STK4b 3.68 0.056

STK4c 14.93 < 0.001 

STK5 72.71 < 0.001 

STK6 93.83 < 0.001 

STK7a 35.49 < 0.001 

STK7b 17.04 < 0.001 

STK8a 33.42 < 0.001 

STK8b 15.56 < 0.001 
34

 Table 13. F-tests of the significance of operator

 Analysis Result

 White Lead Lead Chromate

Spot Test 

Kit F-Statistic P-Value F-Statistic P-Value

STK1 6.21 < 0.001 3.02 0.019

STK2 2.05 0.133 6.28 0.003

STK3 0.04 0.997 17.55 < 0.001 

STK4a 0.96 0.428 14.95 < 0.001 

STK4b 0.96 0.428 13.93 < 0.001 

STK4c 0.96 0.428 0.27 0.896

STK5 2.40 0.051 5.73 < 0.001 

STK6 0.40 0.808 10.23 < 0.001 

STK7a 1.07 0.372 0.74 0.567

STK7b 0.27 0.899 0.44 0.776

STK8a 2.59 0.079 0.37 0.692

STK8b 2.22 0.112 0.32 0.729

5.3 Modeling the Probability of Spot Test Kit Response

The probabilities of positive response, as a function of lead concentration and other covariates, were 

estimated using logistic regression models [32] having the following form:

 log[p/(1-p)] = c0 + M1c1 + M2c2 + c3x + e (Eq 1)

where c0, c1, c2, and c3 are coefficients (or vectors of coefficients), M1, M2, and x are independent 

variables, and e is the error term. The coefficients were estimated by maximum likelihood using the 

glm command in the Splus statistics package*. The constant c0 is the intercept, c1 is the substrate 

effect (omitted for STK2), c2 is a vector of either two or four effects for operator (depending on the 

kit), and c3 is the coefficient associated with concentration. The x is the measured lead 

concentration. The model matrix M1 is a vector of dummy variables for a reactive substrate, and the 

M2 is a matrix for the operator effects.

In performing the modeling, separate regressions were fit for lead pigment type. Preliminary analysis 

had included lead pigment type as a regression covariate, but these models fit poorly for several of 

the kits because of the pronounced difference in response depending on lead pigment type. For each 

spot test kit, overlayer effects were ignored, because they were found insignificant except for STK6 

(Section 5.2.2). However, initial STK6 regression results ignoring overlayer effects appeared 

 

*MathSoft, Inc., Seattle, WA; www.mathsoft.com/splus.
35

sufficient to describe STK6 performance as a function of lead level. Consequently, further modeling 

of the STK6 response was not performed. 

Figures 1 through 8B graphically summarize the results of the regression analyses. Appendix B is a 

tabular summary of response as a function of spot test kit and lead pigment type, and includes all 

estimated coefficients and standard errors. Figures 1 through 8B each provide a summary of the 

results for a single spot test kit, or for a set of data for a kit using a specific procedure; for example, 

the STK7a and STK7b results are in separate figures. As is evident, the figures relate the probability 

of a positive response as a function of lead level, and contain four plots representing the results for 

each of the four combinations of lead pigment type and substrate. Thus, the two plots in a row 

provide a comparison of the effect of lead pigment type for a nonreactive substrate with that for a 

reactive substrate. Similarly, the two plots in a column allow a comparison of the substrate effect for 

white lead with that for lead chromate.

The logistic curves on each plot represent the results of the fit of the model for each of the operators. 

Note in Figures 1 through 8B that some plots show fewer regression curves than operators 

participating in the testing. In these cases, although the data sets for the operators were different, the 

proportions of positive responses were identical at the given lead levels and, thus, the plotted 

regression curves coincided.

The filled circles in Figures 1 through 8B indicate the proportions of positive responses at a given 

lead level. The error bars are the associated (where possible) 95 % binomial confidence intervals 

[33]. The confidence intervals provide a guide to adequacy of fit of the models. Each plot also 

contains two horizontal dashed lines representing the 0.5 and 0.95 probabilities of a positive 

response. Circles are placed on the top horizontal dashed line at the 0.95 probability point for each 

operator. The circles are repeated on each plot just above the x-axis so that the approximate lead 

level of the 0.95 probability point for each operator can be better estimated. Finally, note that the 

lead level region (i.e., < 1 mg/cm2) where remedial action is not required [2,3] is shaded on all plots.

Before discussing the plots, a few comments on the model selection are in order. A simple logistic 

model (Eq 1) was selected rather than the Aenhanced logistic@ model used in the EPA/HUD field 

study [8], because the data adequately fitted the simple logistic model. The enhanced logistic model 

was required in the EPA/HUD study because, in the limit of small concentrations, some of the 

EPA/HUD data sets exhibited long, flat approaches to nonzero values. A simple logistic model does 

not fit such data well, so additional model parameters were added in the EPA/HUD study for left and 

right limits to the probability. Because the lead levels in the present study were selected according to 

the experimental design, the results do not have this behavior. Also, the previous modeling of the 

EPA/HUD field data was done on log concentrations. This was probably necessary because of the 

highly skewed distribution of lead concentrations observed in the field, with many very small lead 

values, and a few very large values. In the present study, the lead levels were selected on 

equally-spaced scales so that there is no gain in taking the logarithm of concentration. If logarithms 

had been used, the interpretation would have been more difficult, because some ad hoc technique 

would have had to be used to treat the many specimens with zero lead levels.
36

Figure 1. Probability of a Positive Response Versus Lead Level for Spot Test Kit 1 (STK1) for 

Each Operator. (The filled circles represent the proportions of positive responses at a 

given lead level, and the error bars are the associated 95 % binomial confidence 

intervals. The two horizontal dashed lines represent the 0.5 and 0.95 probabilities of a 

positive response. Circles are the 0.95 probability point for each operator; they are 

repeated above the horizontal axis for clarity.)
37

Figure 2. Probability of a Positive Response Versus Lead Level for Spot Test Kit 2 (STK2) for 

Each Operator. (The filled circles represent the proportions of positive responses at a 

given lead level, and the error bars are the associated 95 % binomial confidence 

intervals. The two horizontal dashed lines represent the 0.5 and 0.95 probabilities of a 

positive response. Circles are the 0.95 probability point for each operator; they are 

repeated above the horizontal axis for clarity.)
38

Figure 3. Probability of a Positive Response Versus Lead Level for Spot Test Kit 3 (STK3) for 

Each Operator. (The filled circles represent the proportions of positive responses at a 

given lead level, and the error bars are the associated 95 % binomial confidence 

intervals. The two horizontal dashed lines represent the 0.5 and 0.95 probabilities of a 

positive response. Circles are the 0.95 probability point for each operator; they are 

repeated above the horizontal axis for clarity.)
39

Figure 4A. Probability of a Positive Response Versus Lead Level for Spot Test Kit 4 for Each 

Operator; Data Are For the STK4a Procedure. (The filled circles represent the 

proportions of positive responses at a given lead level, and the error bars are the 

associated 95 % binomial confidence intervals. The two horizontal dashed lines 

represent the 0.5 and 0.95 probabilities of a positive response. Circles are the 

0.95 probability point for each operator; they are repeated above the horizontal axis for 

clarity.)
40

Figure 4B. Probability of a Positive Response Versus Lead Level for Spot Test Kit 4 for Each 

Operator; Data Are For the STK4b Procedure. (The filled circles represent the 

proportions of positive responses at a given lead level, and the error bars are the 

associated 95 % binomial confidence intervals. The two horizontal dashed lines 

represent the 0.5 and 0.95 probabilities of a positive response. Circles are the 

0.95 probability point for each operator; they are repeated above the horizontal axis for 

clarity.)
41

Figure 4C. Probability of a Positive Response Versus Lead Level for Spot Test Kit 4 for Each 

Operator; Data Are For the STK4c Procedure. (The filled circles represent the 

proportions of positive responses at a given lead level, and the error bars are the 

associated 95 % binomial confidence intervals. The two horizontal dashed lines 

represent the 0.5 and 0.95 probabilities of a positive response. Circles are the 

0.95 probability point for each operator; they are repeated above the horizontal axis for 

clarity.)
42

Figure 5. Probability of a Positive Response Versus Lead Level for Spot Test Kit 5 (STK5) for 

Each Operator. (The filled circles represent the proportions of positive responses at a 

given lead level, and the error bars are the associated 95 % binomial confidence 

intervals. The two horizontal dashed lines represent the 0.5 and 0.95 probabilities of a 

positive response. Circles are the 0.95 probability point for each operator; they are 

repeated above the horizontal axis for clarity.)
43

Figure 6. Probability of a Positive Response Versus Lead Level for Spot Test Kit 6 (STK6) for 

Each Operator. (The filled circles represent the proportions of positive responses at a 

given lead level, and the error bars are the associated 95 % binomial confidence 

intervals. The two horizontal dashed lines represent the 0.5 and 0.95 probabilities of a 

positive response. Circles are the 0.95 probability point for each operator; they are 

repeated above the horizontal axis for clarity.)
44

Figure 7A. Probability of a Positive Response Versus Lead Level for Spot Test Kit 7 for Each 

Operator; Data Are For the STK7a Procedure. (The filled circles represent the 

proportions of positive responses at a given lead level, and the error bars are the 

associated 95 % binomial confidence intervals. The two horizontal dashed lines 

represent the 0.5 and 0.95 probabilities of a positive response. Circles are the 

0.95 probability point for each operator; they are repeated above the horizontal axis for 

clarity.)
45

Figure 7B. Probability of a Positive Response Versus Lead Level for Spot Test Kit 7 for Each 

Operator; Data Are For the STK7b Procedure. (The filled circles represent the 

proportions of positive responses at a given lead level, and the error bars are the 

associated 95 % binomial confidence intervals. The two horizontal dashed lines 

represent the 0.5 and 0.95 probabilities of a positive response. Circles are the 

0.95 probability point for each operator; they are repeated above the horizontal axis for 

clarity.)
46

Figure 8A. Probability of a Positive Response Versus Lead Level for Spot Test Kit 8 for Each 

Operator; Data Are For the STK8a Procedure. (The filled circles represent the 

proportions of positive responses at a given lead level, and the error bars are the 

associated 95 % binomial confidence intervals. The two horizontal dashed lines 

represent the 0.5 and 0.95 probabilities of a positive response. Circles are the 

0.95 probability point for each operator; they are repeated above the horizontal axis for 

clarity.)
47

Figure 8B. Probability of a Positive Response Versus Lead Level for Spot Test Kit 8 for Each 

Operator; Data Are For the STK8b Procedure. (The filled circles represent the 

proportions of positive responses at a given lead level, and the error bars are the 

associated 95 % binomial confidence intervals. The two horizontal dashed lines 

represent the 0.5 and 0.95 probabilities of a positive response. Circles are the 

0.95 probability point for each operator; they are repeated above the horizontal axis for 

clarity.)
48

5.3.1 Ideal Test Kit Performance for Detecting Lead-Based Paint. In advance of discussing the plots 

in Figures 1 through 8B, it is useful to review the ideal performance [8] of a test kit used for 

detecting lead-based paint. Figure 9 illustrates such ideal performance. Note in the figure that the 

probability of a positive response is essentially 0 and 1 at lead levels less than, and greater than, a 

lead level of 1 mg/cm2, respectively. That is, the transition from a negative to a positive response 

(i.e., inflection point) occurs at about the 1 mg/cm2 lead-based paint lead level. This transition is 

sharp, as denoted by the steepness of the curve at the inflection point. If this sharp transition from a 

negative to a positive response were to occur at lead level significantly less than 1 mg/cm2 (i.e., the 

curve in Figure 9 is shifted to the left), then the test kit might be used as a negative screen for leadbased paint. Similarly, if this transition were to occur at a lead level greater than 1 mg/cm2

 (i.e., the 

curve in Figure 9 is shifted to the right), then the test kit might be used as a positive screen for leadbased paint.

Figure 9. Example of the Ideal Performance of a Spot Test Kit for Determining Lead-Based Paint.

0.0

0.5

1.0

0 1 2 3

Lead Level, mg/cm2

Prob. Pos. Response
49

5.3.2 Effect of Spot Test Kit. In examining the plots in Figures 1 through 8B in relation to Figure 9, 

it is evident that none of the spot test kits gave a response curve that had a sharp transition at about 

the Federal level of 1 mg/cm2 at which a paint is classified as lead-based [2,3]. This finding indicated 

that none of the test kits can be used for determining lead-based paint (i.e., minimum lead level of 

1 mg/cm2). It is also shown in the plots that there were major differences among some test kits in 

their ability to determine lead in the paint specimens. For example, for white lead, the 95 % 

probabilities for STK2, STK3, STK4a/b/c, STK6, STK7a/b and STK8a/b ranged from about 

0.1 mg/cm2 to 1 mg/cm2. In contrast, for STK1 and STK5, these values were in the range of about 

1 mg/cm2 to 3 mg/cm2. 

The steepness of the regression curve over a relatively narrow range of lead levels is a measure of a 

spot test kit=s sensitivity in transitioning from negative to positive responses. Qualitatively, the better 

performing kits have sharper transitions (i.e., steeper slopes at the transition points) and, thus, more 

sharply define a lead level from which differences, negative or positive, can be most readily detected. 

In examining Figures 1 through 8B it can be seen that, in general, those kits that had 95 % 

probabilities of a positive response of <1 mg/cm2 had response curves with steeper slopes at the 

inflection point than those having 95 % probabilities which were >1 mg/cm2. In particular, observe 

that, for white lead, STK2, STK3, STK4a/b/c, STK6, STK7b, and STK8b had quite steep slopes. 

Because the 95 % probabilities generally occurred at lead levels less than 0.5 mg/cm2, these kits 

might be used as negative screens for lead-based paint.

5.3.3 Effect of Lead Pigment Type. For the majority of the spot test kits, lead pigment type had a 

major effect on the results. Generally, white lead was readily detected, whereas lead chromate was 

detected with difficulty. This was consistent with results of the false negative and false positive 

tabulations in Section 5.1 and the analysis given in Section 5.2.3. An example of the effect of lead 

pigment type is illustrated in Figure 3 for STK3. Note, in the two plots for white lead, the very sharp 

(i.e., steep slope) transition from a negative to a positive response with the lead levels of the 95 % 

probability of positive response being about 0.1 mg/cm2. In contrast, the response curves for lead 

chromate make a gradual transition from negative to positive response and the lead levels of the 

95 % probability of positive response are greater than 3.5 mg/cm2.

Another clear illustration of the effect of lead pigment type on kit response is given in Figure 2 for 

STK2. This is the kit in which paint chips were immersed in vinegar overnight before adding the 

sulfide reagent (Table 2). Notice in Figure 2 the reasonably sharp transition from a negative to 

positive response at about 0.2 mg/cm2 with specimens having white lead; whereas the transition was 

extremely gradual for specimens with lead chromate. This finding indicates that vinegar is a more 

effective agent for leaching white lead than lead chromate from paint chips.

5.3.4 Effect of Spot Test Kit Procedure. Comparisons of Figures 4A, 4B, and 4C for STK4, Figures 

7A and 7B for STK7, and Figures 8A and 8B for STK8 illustrate the effect of test procedure on spot 

test kit response. This effect was most apparent for STK4. Observe in Figure 4A that the STK4a 

procedure readily detected white lead, but had difficulty in detecting lead chromate. Moreover, there 

was little improvement in STK4 performance for lead chromate when the STK4b procedure was 

used (Figure 4B). However, the STK4c procedure readily detected the lead chromate. Observe in 

Figure 4C the sharpness of the transition from a negative to positive response with the 95 % 

probability of a positive response occurring at a lead level of about 0.7 mg/cm2. These features were 

clearly absent in the lead chromate plots in Figures 4A and 4B. 
50

The logistic plots for STK4a/b/c for lead chromate are consistent with the false negative tabulations 

discussed in Section 5.1. They also support a suggestion that, when an STK4a result is negative, the 

STK4c test procedure should be performed regardless of the suspected lead pigment type, because of 

the STK4c enhanced sensitivity to lead. Presently, the STK4 manufacturer=s instructions direct the 

user to follow the STK4c test procedure only if the STK4a result is negative and lead chromate is 

suspected. 

The change in test procedure for STK7 and STK8 for detecting lead chromate was not as dramatic 

as for STK4. However, the STK7b and STK8b procedures had the 95 % probability of a positive 

response occurring at lead levels of less than, or equal to, about 1 mg/cm2. This was not always the 

case for the STK7a and STK8a procedures. This is important because, if the 95 % probability of a 

positive response occurs with acceptable certainty at lead levels less than 1 mg/cm2, these kits may 

be useful as negative screens (Section 5.4).

The comparisons of STK7a with STK7b (Figures 7A and 7B), and STK8a with STK8b (Figures 8A 

and 8B) imply that the preferential procedure for using the STK7 and STK8 kits is to deposit the 

sulfide reagent on a chip. Note in Figures 7a and 8a that, in a majority of cases for lead chromate, 

the 95 % probability of a positive response is greater than 1 mg/cm2Csuggesting that STK7a and 

STK8a not be used. This modeling result was in contrast to the false negative and false positive 

results given in Table 10B. Qualitatively, the tabulation in Table 10B suggested that the STK7a/b 

and STK8a/b procedures might be used as negative screens for lead-based paint, because all four 

procedures gave relatively low percents of false negatives for lead chromate. Comments provided by 

the operators during the STK7 and STK8 tests indicated that placing the sulfide reagent on a chip 

that had been removed from the notch was no more difficult than cutting the notch and placing the 

reagent on it. Most operators generally felt they could better judge kit response using the STK7b or 

STK8b procedures than they could using the STK7a or STK8a procedures. 

5.3.5 Effect of Operator. Differences in operator response is roughly estimated by observing the 

spread among the various response curves in a given plot. For white lead, only STK1 was found to 

have a significant operator effect (Table 13). Note in Figure 1 the relatively large spread between 

operator response curves. In contrast, for lead chromate, the majority of the spot test kits (i.e., 

STK1, STK2, STK3, STK4a/b, STK5, and STK6) showed operator effects.

The plots in Figures 1 through 8B illustrate the conclusion from the F-statistic analysis given in 

Section 5.2.4. that the response of those kits that were relatively sensitive to low levels of lead were 

not influenced by operator. For example, compare in Figure 4A that, for STK4 which was sensitive 

to white lead, but performed poorly for lead chromate, the slight spread among operator response 

curves for white lead and the relatively large spread for lead chromate.

5.3.6 Effect of Substrate. In examining Figures 1 through 8B, the feature indicating a substrate 

effect is that the 95 % probability of a positive response occurs at higher (or lower) lead levels for 

the reactive substrate than for the nonreactive substrate. In these cases, the reactivity of the 

substrate has decreased (or increased) the sensitivity of the spot test kit to detecting lead. A clear 

example of a substrate effect is given in Figure 5 for STK5 with white lead. Observe that, for the 

nonreactive substrate, the lead level of the 95 % probability of a positive response always occurred at 

< 2 mg/cm2. On the other hand, for the reactive substrate, the lead level of the 95 % probability of a 

positive response was generally about 3 mg/cm2. 
51

Analysis using the logistic regression model indicated that, for white lead, a statistically significant 

substrate effect was only present for STK5, although the effect was only marginally insignificant for 

STK1. As was found in analyzing the data for other effects, these two spot test kits were again 

those that had the highest percents of false negatives for white lead at the 1 mg/cm2 lead level. For 

lead chromate, substrate effects were found in four of the 12 cases (STK1, STK4b, STK6, and 

STK7a). Thus, included among the eight cases where a substrate effect was not found were STK4c, 

STK7b, and STK8b, which are the three spot test kits that have shown possibility for use as negative 

screens for lead chromate specimens.

In their field investigation using a specific rhodizonate spot test kit (STK4 in the present study), 

Ashley et al. [20] found no substrate effect on response. They attributed this finding to their 

notching procedure, which was to cut through the paint film to the substrate, but not into it, to 

minimize contact between the substrate material and the reagent solution. They assumed that the 

majority of their leaded specimens were white-lead pigmented. Their finding of no substrate effect is 

the same as that in the present study for STK4 for white lead.

In contrast to the results in the present NIST study and the Ashley at al. study [20], the QuanTech 

study [21] found a substrate effect in the field, but only for spot test kits used with reactive 

substrates. The observations were believed to result from increased reagent interaction with the 

reactive field substrate and, possibly, the age and brittleness of the paint for the field samples. The 

QuanTech authors indicated that the performance of test kits on reactive substrates in the field may 

be highly variable and difficult to predict. Consequently, they suggested that spot test kits not be 

used on reactive substrates. However, they did not consider this to be a practical limitation on spot 

test use, because rhodizonate kits can be used with metal substrates, and sulfide kits with plaster 

substrates.

5.3.7 Comparison of NIST Laboratory Data with EPA/HUD Field Data. The spot test kit phase of 

the EPA/HUD field study [8] included field tests conducted with five spot test kits used in the 

present laboratory study. The five test kits common to both were STK3, STK4, STK6, STK7, and 

STK8*. Also, some paint-film substrates were common to both. However, the EPA/HUD study did 

not include lead pigment type or operator as variables. It was of interest to compare applicable 

EPA/HUD field results with the NIST laboratory results for the five common test kits.

 

*

STK5 was also included in the EPA/HUD study [8], but was used according to a procedure by which the 

specimen surface was sanded prior to performing the test. Because the EPA/HUD procedure for STK5 did not include 

cutting a notch in the paint-film, this kit was not considered to be common to both the EPA/HUD and NIST studies.

For STK3, STK4, STK6, STK7, and STK8 test kits, the NIST data were pooled over operators, that 

is, the logistic model (Eq 1) was fit without the M2c2 term. The results are plotted in Figures 10 

through 14. The EPA/HUD data are also plotted in these figures using the enhanced logistic model 

of the EPA/HUD study [8]. The STK4 comparison was made using the NIST STK4b data, because 

the HUD/EPA field data were obtained using a procedure that involved a time delay in judging the 

characteristic color change. Similarly, the STK7 and STK8 comparisons were made using the NIST 

STK7b and STK8b data, because the EPA/HUD data were obtained from procedures that included 

chips. No EPA/HUD data were available for STK6 on the reactive substrate, so this comparison 
52

could not be made. Note in Figures 10 through 14 that the EPA/HUD curves, for a given test kit 

and substrate type, are plotted identically for white lead and lead chromate. This allows for the 

comparisons to be made for both types of lead pigments, although the pigment types in the 

EPA/HUD study were not determined. In reviewing the results of the comparisons in Figures 10 

through 14, it is evident that, for any given lead level somewhat greater than zero, the probability of 

a positive response in the EPA/HUD field study was noticeably less than that observed in the NIST

laboratory study for white lead, but greater than that in the NIST laboratory study for lead chromate. 

That is, the results of the laboratory study, wherein the specimens were of known pigment type 

having extreme solubilities, for the most part bracketed the EPA/HUD field findings.
53

Figure 10. Comparison of the Results of the NIST Laboratory Data With Those of the EPA/HUD 

Field Data [8] for Spot Test Kit 3. (The solid and dashed curves represent the NIST 

and EPA/HUD data, respectively.)
54

Figure 11. Comparison of the Results of the NIST Laboratory Data With Those of the EPA/HUD 

Field Data [8] for Spot Test Kit 4. The Comparison was Made for the STK4b 

Procedure. (The solid and dashed curves represent the NIST and EPA/HUD data,

respectively.)
55

Figure 12. Comparison of the Results of the NIST Laboratory Data With Those of the EPA/HUD 

Field Data [8] for Spot Test Kit 6. The EPA/HUD Study did not Include Data for a 

Reactive Substrate. (The solid and dashed curves represent the NIST and EPA/HUD 

data, respectively.)
56

Figure 13. Comparison of the Results of the NIST Laboratory Data With Those of the EPA/HUD 

Field Data [8] for Spot Test Kit 7. The Comparison was Made for the STK7b 

Procedure. (The solid and dashed curves represent the NIST and EPA/HUD data, 

respectively.)
57

Figure 14. Comparison of the Results of the NIST Laboratory Data With Those of the EPA/HUD 

Field Data [8] for Spot Test Kit 8. The Comparison was Made for the STK8b 

Procedure. (The solid and dashed curves represent the NIST and EPA/HUD data, 

respectively.)
58

 5.4 Upper Confidence Limits on the Lead Level Having a 95 % Probability of a Positive Response

The false negative and false positive data in the present study qualitatively suggested that some of the 

spot test kits might be used as negative screens for lead-based paint. One way of quantitatively 

comparing the utility of spot test kits for practical use as negative screens for lead-based paint is via 

95 % upper confidence limits on the lead level for which there is a 95 % probability of a positive 

response. This approach was taken, but the confidence limits were calculated for an Aaverage@

operator (i.e., an operator with an operator effect of zero). Because relatively few operators were 

included in the study, the between-operator variability could not be estimated very precisely. Thus, 

calculated limits for a Atypical, randomly selected@ operator would be very high lead levels, and not 

useful. For this reason, the compromise approach of an Aaverage@ operator was taken. In 

determining the confidence limits, a non-informative prior Bayesian approach was used, with the 

calculations done using Gibbs sampling and the BUGS (Bayesian analysis Using Gibbs Sampling) 

statistics package∗. Coefficients from the logistic regression fits (Eq 1) were used as starting values 

for the Bayesian analyses [34].

A separate model was fit for each kit, and for white lead and for lead chromate. The substrate effect 

was treated as a fixed effect in the regression model (except for STK2), and the operator effect was 

modeled as random. The posterior distribution of the 95th percentile of the response probability was 

simulated, and the 95th percentile of this distribution estimated.

The results of these calculations are presented in Table 14. For completeness of the data set, the 

results of the calculations for STK2 are included (in the column labeled Anonreactive@) although the 

STK2 test specimens did not have a substrate. The numbers in Table 14 represent the 95 % upper 

confidence bound on the lead level for which there is a 95 % probability of a positive response. If a 

kit produces a negative result, there is 95 % confidence that the lead level of the specimen is below 

the value given in Table 14. Using Table 14 practically, if a value is less than, or equal to, one, then 

the calculation suggests that the spot test kit may be used as a negative screen in relation to the 

Federal level of 1 mg/cm2 at which a paint is classified as lead-based. As a practical consequence of 

this analysis, the following criterion is proposed for acceptance of a spot test kit as a negative screen 

for leaded-based paint: Upon evaluation of spot test kit response, the probability of a negative 

response (with 95 % confidence) at a lead level of 1 mg/cm2 is # 5%. This may be equivalently 

stated that the lead level at which there is a 95 % probability of a positive response (with 95 % 

confidence) should be # 1 mg/cm2.

As seen in Table 14, the values suggest that, for white lead on either a reactive or nonreactive 

substrate, most of the spot test kits can possibly be used as negative screens. Only STK1, STK5, 

STK7a and STK8a had 95 % confidence limits greater than oneCimplying that they are not useful as 

negative screens even for white lead. In contrast, for lead chromate on either a reactive or 

nonreactive substrate, only STK4c, STK7b, and STK8b had 95 % confidence limits that were 1 or 

lessCsuggesting that these three kits have possible use as negative screens for both white lead and 

lead chromate. 

A practical concern regarding acceptance of spot test kits for use as negative screens for lead-based 

paint is kit response in cases where lead is essentially not present in the paint specimen. 

 

*

Spiegelhalter, et al., MRC Biostatistics Unit, Institute of Public Health, Cambridge, UK; available free at 

www.mrc-bsu.com.ac.uk/bugs.
59

 Table 14. Lead level corresponding to a 95 % probability of a positive 

 response for an Aaverage@ operatora

 Lead Level, mg/cm2 

 

 Lead Pigment Type 

White Lead Lead Chromate

 Substrate Typeb Substrate Typeb 

Spot Test Kit Reactive Nonreactive Reactive Nonreactive

STK1 1.9 2.3 6.2 8.4

STK2b--- 0.4c--- 89.8c 

STK3 0.1 0.1 8.1 7.7 

STK4a 0.5 0.4 4.1 3.5 

STK4b 0.5 0.4 3.6 2.8 

STK4c 0.5 0.4 0.6 0.7 

STK5 3.9 2.1 24.6 21.6 

STK6 0.9 0.7 14.3 11.6 

STK7a 1.1 1.2 1.1 1.5 

STK7b 0.3 0.4 0.8 1.0

STK8a 1.9 1.6 1.3 1.5

STK8b 0.5 0.5 0.8 0.9

 

aValues in the table are 95 % upper confidence limits on lead levels corresponding to a

 95 % probability of a positive response for an average operator.

 

b

For sulfide-based and rhodizonate-based kits, the reactive substrates were steel and plaster

 panels, respectively. For both kit types, the nonreactive substrate was wood.

 

cAlthough STK2 tests were performed on samples without substrates, the results of the 

 STK2 confidence limit analysis are presented in this table for completeness of the data set. 

 This allows ready comparison of the STK2 results with those of the other spot test kits.

Specifically, the percent of positive results at the 0 mg/cm2 lead level should be low or, similarly, the 

evaluation of kit response should show that the probability of a positive result at the 0 mg/cm2 lead 

level is acceptable. This is because, in practice, false-positive results may needlessly spur test kit 

users into taking further, but unnecessary, investigative action for the presence of lead. The subject 

of false-positive response and acceptance of spot test kits for use as negative screens for lead-based 

paint is addressed by Cox et al. [21].

With regard to STK4c, the lead chromate finding in Table 14 is consistent with the false negativefalse positive analysis (Section 5.1), indicating the improved performance of STK4 when the STK4c 

procedure was used. With regard to STK7 and STK8, the manufacturers= instructions include 

testing paint chips, but generally for cases where the substrate is metal. These two kits do not 

generally require a chip test in cases where a notch test was negative (as was done in the present 

study). The results in Table 14 suggest that the manufacturers= instructions for STK4, STK7, and 

STK8 be revised to emphasize the STK4c, STK7b, and STK8b procedures included in this study. 

In discussing screens for lead-based paint, it is worth repeating that the present finding that some 

spot test kits may be useful as negative screens for lead-based paint is consistent with that of the 
60

previously-mentioned Ashley et al. [20] field study. They found that a specific rhodizonate-based 

spot test kit (STK4 in the present study) was, for most specimens, capable of detecting lead in paint 

at levels well below the 1 mg/cm2 level with a low percent (about 5 %) of false negatives. 

Consequently, they suggested that rhodizonate-based spot test kits show promise for potential use 

for in-situ lead screening in paint. They also indicated that only a few of their specimens may have 

had lead pigments in a highly insoluble form (e.g., lead chromate).

In summary, the analyses presented herein showed that, if spot test kits are to be used as negative 

screens for lead-based paint that may contain either a readily-dissolved lead pigment or one that is 

difficult to dissolve, then only STK4c, STK7b, and STK8b (of the kits used in the study) met the 

proposed acceptance criterion. On the other hand, if the kits are intended as negative screens only 

for readily dissolved white lead pigments, then the analyses indicated that STK2, STK3, STK4a/b/c, 

STK6, STK7b, and STK8b met the proposed criterion. The difference in response of most of the 

spot test kits to white lead and to lead chromate raises a question regarding the relative extent to 

which paints based on white lead and lead chromate pigments were formerly used in practice. 

Answering this question was beyond the scope of the present study. However, it is important 

because it impacts on decisions regarding which spot test kits may possibly be used as negative 

screens. Obviously, if the amount of lead chromate (or any other difficult-to-dissolve pigment) based 

paint in older housing is considered significant, then the results of this controlled laboratory study 

imply that the choice of spot test kits for negative screens is particularly limited.

As a final comment, the color of the tested paint specimen was not a variable in this study. Thus, the 

effect of paint-film color on spot test kit response in a controlled laboratory study is not known. 

Practical concerns have been expressed that rhodizonate kits have limitations for testing red or pink 

paints and, similarly, sulfide kits have limitations with black and other dark colored paints [6]. The 

results of this study have shown that at least one rhodizonate kit and two sulfide kits may perform as 

negative screens. Thus, if the color of the test paint is of concern, a sulfide kit might be used with 

red paints and the rhodizonate kit with black or dark paints.

6. SUMMARY AND CONCLUSIONS

A laboratory study was conducted to determine the reliability of commercial spot test kits for 

detecting the presence and absence of lead in paint in residences. The majority of the spot tests were 

conducted by certified lead inspectors or risk assessors. Eight spot test kitsCfour sulfide-based and 

four rhodizonate-basedCwere used to conduct more than 3000 tests in an experiment investigating 

the effects of lead level, lead pigment type, operator, paint-film substrate, overlayer paint type, and 

overlayer paint thickness. Test specimens, prepared in the laboratory using either white lead or lead 

chromate pigments mixed in an alkyd paint, had ten lead levels ranging from 0 mg/cm2 to 

3.5 mg/cm2. Most of the specimens consisted of leaded-paint films adhered to substrates considered 

to be reactive or nonreactive to the test kit. A reactive substrate could conceivably affect the 

response of the spot test kit to lead; whereas a nonreactive substrate would not have an effect. For 

all spot test kits, the nonreactive substrate was wood. For sulfide kits, the reactive substrate was 

steel and, for rhodizonate kits, it was gypsum plaster. One sulfide kit was designed to test paint 

chips and, thus, the specimens for this kit were not adhered to substrates when tested. All leadedpaint films were overlaid with multiple applications of a nonleaded paint, either a latex or an oil. 

These overlayers had film thicknesses that were either thin (about 0.2 mm or 0.009 in) or thick 

(about 1 mm or 0.04 in). Five operators, three of whom were certified lead inspectors or risk 

assessors, and two of whom had completed risk assessor training, participated in the testing. All 
61

operators were trained to conduct the spot tests using detailed protocols developed from the kit 

manufacturers= instructions.

The analyses and discussion of the spot test results addressed: (1) false negatives and false positives 

obtained during testing, (2) effects of the experimental design factors (e.g., lead pigment type, 

operator, substrate, and overlayer type and thickness) on test kit response, and (3) logistic modeling 

relating the probability of a positive response to lead level. The logistic regression models were 

found to fit the response data well. They provided a basis for calculating the 95 % upper confidence 

limits on the lead level for which there was a 95 % probability of a positive response. In turn, these 

calculations provided a basis for conclusions as to which spot test kits might possibly be used as 

negative screens for lead-based paint. The main conclusions were as follows:

 $ Currently available spot test kits cannot be used to determine lead-based paint, which is defined 

as a paint having lead at levels equal to, or greater than, 1 mg/cm2. This finding was consistent 

with conclusions from several previously published field studies. As was found in the field 

studies, the spot test kits in this controlled laboratory study generally gave relatively high 

percents of false positives at the lead-based paint level of 1 mg/cm2. That is, the spot test kits 

were generally sensitive to lead in paint at much lower levels.

 $ Major differences existed among the responses of the spot test kits. Based on the results of the 

study, a criterion against which a spot test kit may be considered as acceptable for use as a 

negative screen for the presence of lead-based paint was proposed. This criterion is: Upon 

evaluation of spot test kit response, the probability of a negative response (with 95 % 

confidence) at a lead level of 1 mg/cm2 is # 5%. Equivalently, the lead level at which there is a 

95 % probability of a positive response (with 95 % confidence) should be # 1 mg/cm2. Judged 

against this criterion, some of the spot test kits in the study may be acceptable for use as negative 

screens for lead-based paint. However, qualification as a negative screen depends upon the type 

of lead pigment in the test specimen. For white lead specimens, six kits—three sulfide-based 

(STK2, STK7b, and STK8b) and three rhodizonate-based (STK3, STK4a/b/c, and STK6)—gave 

percents of false negatives of # 2 % and met the proposed negative screen criterion for leadbased paint. For lead chromate specimens, three of these six kits—two sulfide-based (STK7b 

and STK8b) and one rhodizonate-based (STK4c)—had percents of false negatives of # 2 % and 

met the proposed negative screen criterion.

 $ Test procedure had an important effect on spot test kit response. The three spot test kits 

(STK4c, STK7b, and STK8b) most likely to qualify as negative screens for lead-based paint 

having either white lead or lead chromate pigments had alternative steps in their test procedures. 

The test kit responses depended on which procedure was used. 

 $ The type of lead pigment had a significant effect on the response of the spot test kits. White lead 

was readily detected at low lead levels, whereas lead chromate was not. The significance of lead 

pigment type was not unexpected, as the two pigments were incorporated into the design as 

being relatively readily dissolved (white lead) and dissolved with difficulty (lead chromate). The 

observed pigment-type effect supported the assumption that the presence of a lead pigment that 

is relatively difficult to dissolve may be difficult to detect using spot test kits.

 $ The other factorsCoverlayer type, overlayer thickness, operator, and substrateCdid not generally 

show significant effects in cases where the spot test kits appeared to be candidates for use as 

negative screens for lead-based paint. The findings have practical benefits because these factors 
62

(perhaps with the exception of operator) are uncontrollable in practice. Based on this laboratory 

study, variations of these factors in practice might not be expected to affect spot test kit response 

when the kits are used properly. The lack of a substrate effect was in contrast to the QuanTech 

finding of a substrate effect for both rhodizonate and sulfide kits for field samples on reactive 

substrates. However, the QuanTech authors have suggested that the substrate effect observed in 

the field should not be a practical limitation on spot test use, because rhodizonate kits can be 

used with metal substrates, and sulfide kits with plaster substrates.

 $ An exception to the lack of an overlayer type effect was present for one rhodizonate kit (STK6). 

In this case, the test procedure included grinding paint chips in a small vial and then extracting 

lead with a leaching agent. This spot test kit was more responsive to specimens having latex 

paint overlayers than those with oil-based paint overlayers. Apparently, latex overlayers were 

more readily separated from the leaded-paint film during the grinding step than oil-based paint 

overlayers. Thus, the lead was more accessible to the leaching agent for latex paint overlayers 

than for oil-based paint overlayers. The finding illustrates that the more the spot test kit reagent 

can access lead in the paint, then the more likely the spot test kit will give a positive response.

 $ A comparison of the NIST laboratory data with those from an EPA/HUD field study (for kits 

that were common to both) showed that the results of the laboratory study, having specimens 

prepared with relatively soluble white lead and difficult-to-dissolve lead chromate, generally 

bracketed the EPA/HUD field findings. For any lead level somewhat greater than zero, the 

NIST laboratory study had greater probabilities of a positive response than the EPA/HUD field 

study for white lead specimens and smaller probabilities of a positive response than the 

EPA/HUD field study for lead chromate. 

7. ACKNOWLEDGMENTS

This study was sponsored by the U. S. Department of Housing and Urban Development (HUD). 

The authors acknowledge the support and encouragement of Peter Ashley, David Jacobs, and 

Warren Friedman, who are with the HUD Office of Lead Hazard Control. Thanks are expressed to 

those whose assistance contributed significantly to the conduct of the study. John Jones, QuanTech, 

prepared the paint panels and chips. Jack Lee, NIST, readied the testing laboratory and assisted in 

paint sample preparation. Connie Reese, QuanTech, and Amanda Grasso, NIST, helped to prepare 

paint samples for laboratory analyses of lead content. James Filliben, NIST, assisted with the 

experimental design. NIST colleagues, Geoffrey Frohnsdorff, Latiff Hussain, Jonathan Martin, Tinh 

Nguyen, and Arnold Rosenberg, provided reviews of the report. Phyllis Madigan, Massachusetts 

Department of Public Health, State Laboratory Institute, provided sulfide analyses of sulfide-based 

spot test kits. Finally, the authors express their appreciation to those manufacturers= representatives 

who reviewed the test protocols developed for each spot test kit.

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63

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the Lead-Based Paint Poisoning Prevention Act at Navy Facilities,@ Technical Note N-1533, 

Civil Engineering Laboratory, Port Hueneme, CA (October 1978), 47 pages.

 [13] McKnight, Mary E., Byrd, W. Eric, Roberts, Willard E., and Lagergren, Eric S., AMethods for 

Measuring Lead Concentrations in Paint Films,@ NISTIR 89-4209, National Institute of 

Standards and Technology, Gaithersburg, MD (October 1989), 122 pages.

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64

 [15] Adler, Robert G., ALead Test Kits,@ OSHA Technical Center, Salt Lake City, UT (September 

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 [16] Harper, Sharon L., Gutknecht, William F., and Beard, Michael E., APerformance Evaluations 

of Measurement Methods Used for Lead-Based paint Contaminated Media: Current Status,@ in 

Lead in Paint, Soil, and Dust: Health Risks, Exposure Studies, Control Measures, 

Measurement Methods, and Quality Assurance, ASTM STP 1226, Beard, Michael E. and Iske, 

S.D. Allen, Eds., American Society for Testing and Materials, Philadelphia, PA (1995), pp. 

191-206.

 [17] Hutter, Gary and Moshman, Diane, AComparisons among Lead Paint Field Screening Test 

Methods,@ J. Hazardous Materials, Vol. 40 (1995), pp. 1-13.

 [18] Pinto, Michael A., AFurther Evidence for the Effectiveness of Chemical Testing for Lead,@

Lead Perspectives (October 1996), pp. 19-21.

 [19] Reames, Ginger, Brumis, Simone, Lance, Larrie L., and Schwartzberg, Steve, ALead Paint Test 

Method Comparison Study,@ California Department of Health Services, Emeryville, CA 

(September 1997), 56 pages.

 [20] Ashley, Kevin, Hunter, Meredith, Tait, Lynn H., Dozier, Judy, Seaman, J. Lee, and Berry, 

Peter F., AField Evaluation of On-Site Techniques for the Measurement of Lead in Paint 

Films,@ Field Analytical Chemistry and Technology, Vol. 2, No. 1 (1998), pp. 39-50.

[21] Cox, David, Schmehl, Richard, Dewalt, Gary, Ashley, Peter, and Friedman, Warren, AThe Use 

of Manufactured Samples for Evaluating Spot Test Kits for Detecting Lead in Household 

Paints,@ HUD Technical Report, U.S. Department of Housing and Urban Development, 

Washington, DC (in preparation).

[22] Cox, David, Schmehl, Richard, Dewalt, Gary, Ashley, Peter, and Friedman, Warren, AGuidance 

for the Evaluation of Spot Test Kits for Detecting the Presence of Lead in Household Paints,@

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DC (in preparation).

 [23] Dunn, Edward J. Jr., ALead Pigments,@ Chapter 7 in Pigments, Treatise on Coatings, 

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(1976).

 [24] Holley, Clifford Dyer and Ladd, E.F., Analysis of Mixed Paints, John Wiley & Sons, New 

York (1908).

 [25] Mattiello, Joseph J., Ed., Protective and Decorative Coatings, Vol. II, John Wiley & Sons, 

New York, NY (1942).

 [26] Parker, Dean H., Principles of Surface Coating Technology, Interscience Publishers, New 

York, NY (1965).

 [27] APreparation and Evaluation of Sodium Sulfide Solution for Lead Spot Test Kits,@

Environmental Lead Laboratory, State Laboratory Institute, Massachusetts Department of 

Public Health (1998).
66

 [28] National Lead Laboratory Accreditation Program, Federal Register, Vol. 61, No. 221 

(November 14, 1996), pp. 58408-58409.

 [29] Estes, E.D., Williams, E.E., and Gutknecht, W.F., AOptions for a Test Kit Certification 

Program,@ Research Triangle Institute, Research Triangle Park, NC (February 1991).

 [30] Draper, Norman and Smith, Harry, Applied Regression Analysis, 2nd edition, John Wiley and 

Sons, New York, NY (1981), p. 439.

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Chapman and Hall, New York, NY (1996).
A1

APPENDIX A. ADDITIONAL TABULATIONS OF FALSE NEGATIVES AND FALSE

 POSITIVES

This Appendix provides tabulations of false negatives and false positives at the 0.5 mg/cm2 and 

0.7 mg/cm2 lead levels. They are provided for those readers who have interest in the percents of 

false negatives and false positives at lead levels other than 0 mg/cm2 and 1.0 mg/cm2 that were 

discussed in the main text.

 Table A1. False negatives and false positivesa at a lead level of 0.5 mg/cm2

Lead

Pigment

Type Kit

Total

No. of

Observations

 False Negatives 

No. % 

Total

No. of

Observations

 False Positives 

No. % 

STK1 80 18 23 160 54 34

STK2 48 0 0 96 73 76

STK3 80 0 0 160 97 61

STK4a 80 2 3 160 90 56

STK4b 80 2 3 160 90 56

STK4c 80 2 3 160 91 57

STK5 80 20 25 160 60 38

STK6 80 4 5 160 57 36

STK7a 80 7 9 160 63 39

STK7b 80 0 0 160 86 54

STK8a 48 12 25 96 28 29

White

Lead

STK8b 48 0 0 96 43 45

STK1 160 83 52 80 15 19

STK2 99 41 41 45 23 51

STK3 165 102 62 75 5 7

STK4a 165 50 30 75 11 15

STK4b 165 40 24 75 11 15

STK4c 165 2 1 75 18 24

STK5 165 132 80 75 4 5

STK6 165 132 80 75 4 5

STK7a 160 7 4 80 20 25

STK7b 160 3 2 80 26 33

STK8a 96 9 9 48 10 21

Lead

Chromate

STK8b 96 3 3 48 14 29

 

a

A false negative has been defined as a test result that is negative for lead, but the true value is above

 the selected lead level [8]. Conversely, a false positive is a test result that is positive for lead, but the 

 true value is below the selected lead level.
A2

 Table A2. False negatives and false positivesa at a lead level of 0.7 mg/cm2

Lead

Pigment

Type Kit

Total

No. of

Observations

 False Negatives 

No. % 

Total

No. of

Observations

 False Positives 

No. % 

STK1 60 10 17 180 66 37

STK2 39 0 0 105 82 78

STK3 60 0 0 180 117 65

STK4a 60 1 2 180 109 61

STK4b 60 1 2 180 109 61

STK4c 60 1 2 180 110 61

STK5 60 14 23 180 74 41

STK6 60 1 2 180 74 41

STK7a 60 3 5 180 79 44

STK7b 60 0 0 180 106 59

STK8a 36 5 14 108 33 31

White

Lead

STK8b 36 0 0 108 55 51

STK1 140 71 51 100 23 23

STK2 87 35 40 57 29 51

STK3 145 89 61 95 12 13

STK4a 145 42 29 95 23 24

STK4b 145 33 23 95 24 25

STK4c 145 1 1 95 37 39

STK5 145 116 80 95 8 8

STK6 145 114 79 95 6 6

STK7a 140 6 4 100 39 39

STK7b 140 2 1 100 45 45

STK8a 84 5 6 60 18 30

Lead

Chromate

STK8b 84 1 1 60 24 40

 

a

A false negative has been defined as a test result that is negative for lead, but the true value is above

 the selected lead level [8]. Conversely, a false positive is a test result that is positive for lead, but the 

 true value is below the selected lead level.
B1

APPENDIX B. TABULATIONS OF THE RESULTS OF MODELING TEST KIT RESPONSE 

VERSUS LEAD LEVEL

This Appendix tabulates a summary of the results of the modeling of the probability of a positive 

response versus lead level (Section 5.3). The tabulations are according to test kit (i.e., STK1 through 

STK8) and lead pigment type (i.e., white lead and lead chromate). Each summary page gives the 

following information:

Coefficients:

Included here are the model coefficients, the corresponding standard errors, and the t-values, which 

are the ratios of the coefficients to the standard errors.

Null Deviance and Residual Deviance:

These parameters are useful for assessing goodness of fit.

Correlation of Coefficients:

These matrices are given for completeness of the analysis summaries, but were not discussed in the 

text.
B2

Kit 1, White Lead

Coefficients:

 Value Std. Error t value 

(Intercept) -1.36275317 0.2510672 -5.4278429

oper.w1 0.09249875 0.3208699 0.2882749 

oper.w2 -2.03519621 0.4565424 -4.4578470

oper.w3 1.15978049 0.3279986 3.5359315 

oper.w4 0.56908888 0.3185882 1.7862838

conc.w 2.81525229 0.4990840 5.6408381 

subs.w -0.48183649 0.1658948 -2.9044694

Null Deviance: 332.4439 on 239 degrees of freedom

Residual Deviance: 222.3124 on 233 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.w1 oper.w2 oper.w3 oper.w4 conc.w 

oper.w1 -0.1016292

oper.w2 0.4369074 -0.3333857 

oper.w3 -0.2189427 -0.1711617 -0.3855269

oper.w4 -0.1748419 -0.1721133 -0.3591690 -0.1416527 

conc.w -0.7295886 0.0527451 -0.3376222 0.2134241 0.1355702

subs.w 0.0695350 0.0063270 0.1052444 -0.1060210 -0.0425805 -0.1149824
B3

Kit 1, Lead Chromate

Coefficients:

 Value Std. Error t value 

(Intercept) -1.4489221 0.2560654 -5.6584062

oper.y1 -0.2430896 0.3025390 -0.8034986 

oper.y2 -0.7333256 0.3225611 -2.2734473

oper.y3 0.5068933 0.2920885 1.7354103 

oper.y4 0.7126115 0.2932821 2.4297815

conc.y 0.8489410 0.1759965 4.8236237 

subs.y -0.6400111 0.1528018 -4.1885060

Null Deviance: 319.5228 on 239 degrees of freedom

Residual Deviance: 263.7064 on 233 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.y1 oper.y2 oper.y3 oper.y4 conc.y 

oper.y1 0.0324432

oper.y2 0.1317794 -0.2683962 

oper.y3 -0.0931776 -0.2396354 -0.2718523

oper.y4 -0.1190714 -0.2401426 -0.2753511 -0.2025401 

conc.y -0.8078911 -0.0361860 -0.1021392 0.0783474 0.1089637

subs.y 0.1502625 0.0340557 0.0848762 -0.0672471 -0.0966374 -0.1457449
B4

Kit 2, White Lead

Coefficients:

 Value Std. Error t value 

(Intercept) -2.1804607 0.4326228 -5.0400965

oper.w1 -0.1473719 0.3872902 -0.3805206 

oper.w2 0.2947438 0.3844623 0.7666391

conc.w 10.5928333 1.8544102 5.7122384

Null Deviance: 191.5223 on 143 degrees of freedom

Residual Deviance: 86.13381 on 140 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.w1 oper.w2

oper.w1 0.0520290

oper.w2 -0.1048234 -0.4963491 

conc.w -0.7771057 -0.0575497 0.1159460
B5

Kit 2, Lead Chromate

Coefficients:

 Value Std. Error t value 

(Intercept) -9.544071 6.8545425 -1.3923717 

oper.y1 -5.717279 13.2656615 -0.4309833

oper.y2 2.858639 6.6517885 0.4297550 

conc.y 2.087765 0.6529088 3.1976368

Null Deviance: 49.8631 on 143 degrees of freedom

Residual Deviance: 27.77106 on 140 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.y1 oper.y2 

oper.y1 0.9687697

oper.y2 -0.9660087 -0.9971500 

conc.y -0.2490057 -0.0124999 0.0124643
B6

Kit 3, White Lead

Coefficients:

 Value Std. Error t value 

(Intercept) -8.441500 1.9339947 -4.3648001

oper.w1 1.812415 1.5395534 1.1772346 

oper.w2 1.812415 1.5395534 1.1772346

oper.w3 -1.208277 1.2135703 -0.9956383 

oper.w4 -1.208277 1.2135703 -0.9956383

conc.w 118.407095 25.5725027 4.6302506 

subs.w 1.518890 0.8106739 1.8736136

Null Deviance: 276.3147 on 239 degrees of freedom

Residual Deviance: 9.125448 on 233 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.w1 oper.w2 oper.w3 oper.w4 conc.w 

oper.w1 -0.2601287 

oper.w2 -0.2601287 -0.1795750

oper.w3 0.2200021 -0.3469345 -0.3469345 

oper.w4 0.2200021 -0.3469345 -0.3469345 -0.0598738

conc.w -0.9364413 0.3132712 0.3132712 -0.2649470 -0.2649470 

subs.w -0.3896507 0.0402728 0.0402728 -0.0340605 -0.0340605 0.5208314
B7

Kit 3, Lead Chromate

Coefficients:

 Value Std. Error t value 

(Intercept) -2.5236989 0.3494064 -7.222819

oper.y1 2.0026503 0.3359171 5.961740 

oper.y2 -1.7852240 0.5166200 -3.455585

oper.y3 -0.3739769 0.3667327 -1.019753 

oper.y4 0.8842392 0.3213585 2.751567

conc.y 1.0014342 0.1935945 5.172845 

subs.y 0.2139908 0.1774439 1.205963

Null Deviance: 286.1155 on 239 degrees of freedom

Residual Deviance: 204.6798 on 233 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.y1 oper.y2 oper.y3 oper.y4 conc.y 

oper.y1 -0.4265081 

oper.y2 0.3556155 -0.3923925

oper.y3 0.0192313 -0.1518673 -0.3483555 

oper.y4 -0.2810453 0.0137474 -0.3759985 -0.1477675

conc.y -0.8318418 0.3429577 -0.1857015 -0.0698450 0.1647647 

subs.y -0.2062771 0.0800818 -0.0383997 -0.0186834 0.0333789 0.1819493
B8

Kit 4a, White Lead

Coefficients:

 Value Std. Error t value 

(Intercept) -1.6311033 0.3210389 -5.0807035

oper.w1 0.5424358 0.4222753 1.2845549 

oper.w2 -0.8251505 0.4504299 -1.8319175

oper.w3 -0.5816834 0.4393685 -1.3239077 

oper.w4 0.1069457 0.4225396 0.2531023

conc.w 12.3185812 1.9078078 6.4569300 

subs.w 0.3748677 0.2273511 1.6488493

Null Deviance: 293.2149 on 239 degrees of freedom

Residual Deviance: 136.7059 on 233 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.w1 oper.w2 oper.w3 oper.w4 conc.w 

oper.w1 -0.1213784

oper.w2 0.1788882 -0.2770057 

oper.w3 0.1193589 -0.2649106 -0.2323634

oper.w4 -0.0409804 -0.2276917 -0.2623978 -0.2571303 

conc.w -0.7333541 0.1421814 -0.2090339 -0.1412369 0.0439170

subs.w -0.3454980 0.0329095 -0.0474296 -0.0338954 0.0066425 0.2841521
B9

Kit 4a, Lead Chromate

Coefficients:

 Value Std. Error t value 

(Intercept) -1.6056028 0.2906222 -5.5247083

oper.y1 1.3653126 0.3466097 3.9390491 

oper.y2 -1.5845883 0.3650811 -4.3403733

oper.y3 -0.8767432 0.3301881 -2.6552838 

oper.y4 -0.1307868 0.3145738 -0.4157586

conc.y 1.5722936 0.2336738 6.7285818 

subs.y 0.4116489 0.1709843 2.4075247

Null Deviance: 332.1104 on 239 degrees of freedom

Residual Deviance: 224.4295 on 233 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.y1 oper.y2 oper.y3 oper.y4 conc.y 

oper.y1 -0.2109410

oper.y2 0.2948437 -0.3358094 

oper.y3 0.1370830 -0.2840340 -0.1897881

oper.y4 -0.0286215 -0.2299576 -0.2374611 -0.2174719 

conc.y -0.8214856 0.2658138 -0.3145753 -0.1802346 -0.0090692

subs.y -0.2620000 0.0940479 -0.1085483 -0.0633909 -0.0051266 0.2558601
B10

Kit 4b, White Lead

Coefficients:

 Value Std. Error t value 

(Intercept) -1.6311033 0.3210389 -5.0807035 

oper.w1 0.5424358 0.4222753 1.2845549

oper.w2 -0.8251505 0.4504299 -1.8319175 

oper.w3 -0.5816834 0.4393685 -1.3239077

oper.w4 0.1069457 0.4225396 0.2531023 

conc.w 12.3185812 1.9078078 6.4569300

subs.w 0.3748677 0.2273511 1.6488493

Null Deviance: 293.2149 on 239 degrees of freedom

Residual Deviance: 136.7059 on 233 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.w1 oper.w2 oper.w3 oper.w4 conc.w 

oper.w1 -0.1213784 

oper.w2 0.1788882 -0.2770057

oper.w3 0.1193589 -0.2649106 -0.2323634 

oper.w4 -0.0409804 -0.2276917 -0.2623978 -0.2571303

conc.w -0.7333541 0.1421814 -0.2090339 -0.1412369 0.0439170 

subs.w -0.3454980 0.0329095 -0.0474296 -0.0338954 0.0066425 0.2841521
B11

Kit 4b, Lead Chromate

Coefficients:

 Value Std. Error t value 

(Intercept) -1.7664972 0.3151361 -5.6055058

oper.y1 1.2605363 0.3672616 3.4322571 

oper.y2 -1.9464044 0.4027584 -4.8326841

oper.y3 -0.2977721 0.3372528 -0.8829346 

oper.y4 -0.4361002 0.3389206 -1.2867326

conc.y 2.0180261 0.2851039 7.0782128 

subs.y 0.5884613 0.1863793 3.1573318

Null Deviance: 328.4312 on 239 degrees of freedom

Residual Deviance: 202.1229 on 233 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.y1 oper.y2 oper.y3 oper.y4 conc.y 

oper.y1 -0.2272930

oper.y2 0.3740769 -0.3525461 

oper.y3 0.0138009 -0.2412518 -0.2096741

oper.y4 0.0458495 -0.2517148 -0.1947915 -0.2105939 

conc.y -0.8296673 0.2777952 -0.4119777 -0.0520826 -0.0883697

subs.y -0.3250772 0.1167252 -0.1768140 -0.0216312 -0.0362746 0.3369428
B12

Kit 4c, White Lead

Coefficients:

 Value Std. Error t value 

(Intercept) -1.52569090 0.3122666 -4.8858602

oper.w1 0.69914204 0.4199514 1.6648167 

oper.w2 -0.84842893 0.4450508 -1.9063643

oper.w3 -0.61120851 0.4343032 -1.4073314 

oper.w4 0.06135336 0.4178145 0.1468435

conc.w 11.90263652 1.8578956 6.4065154 

subs.w 0.30423930 0.2223688 1.3681747

Null Deviance: 291.5004 on 239 degrees of freedom

Residual Deviance: 139.2484 on 233 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.w1 oper.w2 oper.w3 oper.w4 conc.w 

oper.w1 -0.1400064 

oper.w2 0.1781837 -0.2819451

oper.w3 0.1207004 -0.2684036 -0.2294360 

oper.w4 -0.0338179 -0.2276012 -0.2599229 -0.2555191

conc.w -0.7237190 0.1707958 -0.2122192 -0.1461507 0.0346331 

subs.w -0.3171832 0.0351755 -0.0404881 -0.0296083 0.0031934 0.2508397
B13

Kit 4c, Lead Chromate

Coefficients:

 Value Std. Error t value 

(Intercept) -2.5778262 0.5164363 -4.9915665

oper.y1 0.5822743 0.7405648 0.7862570 

oper.y2 -0.1271855 0.7776655 -0.1635478

oper.y3 -0.9101776 0.8186281 -1.1118328 

oper.y4 -0.1271855 0.7776655 -0.1635478

conc.y 9.5613154 1.6093192 5.9412175 

subs.y -0.2814054 0.3856290 -0.7297309

Null Deviance: 267.7013 on 239 degrees of freedom

Residual Deviance: 51.73587 on 233 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.y1 oper.y2 oper.y3 oper.y4 conc.y 

oper.y1 -0.1910693

oper.y2 0.0561476 -0.2479740 

oper.y3 0.2390222 -0.2967274 -0.2437658

oper.y4 0.0561476 -0.2479740 -0.2711067 -0.2437658 

conc.y -0.6640143 0.2223067 -0.0411471 -0.3240393 -0.0411471

subs.y -0.1046787 -0.0109302 -0.0018779 0.0233438 -0.0018779 -0.0788685
B14

Kit 5, White Lead

Coefficients:

 Value Std. Error t value 

(Intercept) -1.1420205 0.2431321 -4.6971200

oper.w1 0.1234607 0.3255567 0.3792295 

oper.w2 -0.2703599 0.3267309 -0.8274699

oper.w3 -0.4026677 0.3284992 -1.2257799 

oper.w4 -0.6720811 0.3342511 -2.0107075

conc.w 1.8389256 0.3724323 4.9376100

subs.w 1.3217702 0.1955425 6.7595048

Null Deviance: 332.7106 on 239 degrees of freedom

Residual Deviance: 222.6637 on 233 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.w1 oper.w2 oper.w3 oper.w4 conc.w 

\oper.w1 -0.0234052

oper.w2 0.0286565 -0.2401542 

oper.w3 0.0478629 -0.2413588 -0.2324778

oper.w4 0.0902547 -0.2454535 -0.2322303 -0.2289999 

conc.w -0.7116047 0.0146776 -0.0390115 -0.0569376 -0.0936648

subs.w -0.4747806 0.0136787 -0.0724531 -0.0979361 -0.1441788 0.4337426
B15

Kit 5, Lead Chromate

Coefficients:

 Value Std. Error t value 

(Intercept) -2.8608463 0.4007503 -7.1387256

oper.y1 0.8821125 0.3812244 2.3138931 

oper.y2 -0.3667745 0.4885731 -0.7507055

oper.y3 -0.1109643 0.4568446 -0.2428929 

oper.y4 -1.8479362 0.8108532 -2.2790023

conc.y 0.5557294 0.1943368 2.8596193 

subs.y 0.3855487 0.2043058 1.8871155

Null Deviance: 206.3371 on 239 degrees of freedom

Residual Deviance: 171.5106 on 233 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.y1 oper.y2 oper.y3 oper.y4 conc.y 

oper.y1 -0.3306883

oper.y2 -0.0340191 -0.0717824 

oper.y3 -0.1015288 -0.0188681 -0.1520064

oper.y4 0.4274186 -0.4950682 -0.4606204 -0.4665471 

conc.y -0.7429199 0.0572509 -0.0365153 -0.0253045 -0.0467767

subs.y -0.2837647 0.0325136 -0.0221996 -0.0159935 -0.0270795 0.1990343
B16

Kit 6, White Lead

Coefficients:

 Value Std. Error t value 

(Intercept) -2.28271898 0.3366412 -6.7808670

oper.w1 -0.48906890 0.3996509 -1.2237402

oper.w2 0.26172511 0.3796488 0.6893874

oper.w3 0.08221315 0.3823295 0.2150322

oper.w4 0.43731040 0.3782489 1.1561446

conc.w 7.59510499 1.0811972 7.0247175

subs.w 0.60052472 0.2132966 2.8154449

Null Deviance: 329.8884 on 239 degrees of freedom

Residual Deviance: 168.2766 on 233 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.w1 oper.w2 oper.w3 oper.w4 conc.w 

oper.w1 0.1193364 

oper.w2 -0.0667350 -0.2619113

oper.w3 -0.0243329 -0.2605846 -0.2378031 

oper.w4 -0.1060110 -0.2631845 -0.2237147 -0.2328847

conc.w -0.8142665 -0.1193474 0.0669697 0.0230995 0.1085301 

subs.w -0.4193873 -0.0432574 0.0241429 0.0080342 0.0398858 0.4034980
B17

Kit 6, Lead Chromate

Coefficients:

 Value Std. Error t value 

(Intercept) -3.5533449 0.4852844 -7.322191 

oper.y1 -0.6357992 0.5583144 -1.138783

oper.y2 -0.6357992 0.5583144 -1.138783 

oper.y3 1.1976612 0.4034159 2.968800

oper.y4 1.9109444 0.3960849 4.824583

conc.y 0.8422438 0.2186348 3.852286

subs.y 0.7533830 0.2356750 3.196704

Null Deviance: 206.3371 on 239 degrees of freedom

Residual Deviance: 148.2332 on 233 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.y1 oper.y2 oper.y3 oper.y4 conc.y 

oper.y1 0.0536150 

oper.y2 0.0536150 -0.2298895

oper.y3 -0.3872963 -0.1162826 -0.1162826 

oper.y4 -0.5219782 -0.1071460 -0.1071460 0.2355157

conc.y -0.7870674 -0.0734037 -0.0734037 0.1342675 0.2502228 

subs.y -0.4283817 -0.0535577 -0.0535577 0.0881546 0.1844558 0.2946974
B18

Kit 7a, White Lead

Coefficients:

 Value Std. Error t value 

(Intercept) -1.24789770 0.2397445 -5.20511411 

oper.w1 0.24506593 0.3256436 0.75255878

oper.w2 -0.29022988 0.3322600 -0.87350235 

oper.w3 0.63778134 0.3283704 1.94226183

oper.w4 -0.01897972 0.3273825 -0.05797416 

conc.w 4.59230556 0.7302960 6.28827950

subs.w -0.17935896 0.1651177 -1.08624890

Null Deviance: 328.4312 on 239 degrees of freedom

Residual Deviance: 219.6261 on 233 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.w1 oper.w2 oper.w3 oper.w4 conc.w 

oper.w1 -0.0520838

oper.w2 0.0449045 -0.2491494 

oper.w3 -0.0975308 -0.2300101 -0.2523516

oper.w4 -0.0086125 -0.2406110 -0.2493214 -0.2398661 

conc.w -0.7249254 0.0546375 -0.0549672 0.1211055 0.0026965

subs.w -0.0426301 -0.0084133 0.0119862 -0.0266185 0.0025227 0.0037335
B19

Kit 7a, Lead Chromate

Coefficients:

 Value Std. Error t value 

(Intercept) -1.30990630 0.3099309 -4.2264462 

oper.y1 0.08857063 0.4343508 0.2039150

oper.y2 0.56694322 0.4437499 1.2776187 

oper.y3 0.32613661 0.4380523 0.7445152

oper.y4 -0.14619429 0.4326567 -0.3378991 

conc.y 3.79834687 0.5320681 7.1388361

subs.y -0.70789912 0.2237735 -3.1634621

Null Deviance: 284.239 on 239 degrees of freedom

Residual Deviance: 136.017 on 233 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.y1 oper.y2 oper.y3 oper.y4 conc.y 

oper.y1 -0.0187903 

oper.y2 -0.0893257 -0.2479923

oper.y3 -0.0560810 -0.2491260 -0.2393729 

oper.y4 0.0211768 -0.2497742 -0.2560868 -0.2532618

conc.y -0.7092349 0.0279227 0.1453851 0.0888448 -0.0358893 

subs.y 0.0654757 -0.0071285 -0.0691156 -0.0373876 0.0213456 -0.2101573
B20

Kit 7b, White Lead

Coefficients:

 Value Std. Error t value 

(Intercept) -1.70624487 0.3259626 -5.2344801

oper.w1 -0.04463219 0.4431382 -0.1007185 

oper.w2 -0.54687477 0.4557300 -1.1999974

oper.w3 0.19862514 0.4405957 0.4508104 

oper.w4 0.43751402 0.4402536 0.9937772

conc.w 14.35786754 2.1657840 6.6294088 

subs.w -0.38299780 0.2224430 -1.7217793

Null Deviance: 296.5251 on 239 degrees of freedom

Residual Deviance: 129.0506 on 233 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.w1 oper.w2 oper.w3 oper.w4 conc.w 

oper.w1 0.0070003

oper.w2 0.1218043 -0.2532509 

oper.w3 -0.0460784 -0.2476723 -0.2598660

oper.w4 -0.0940642 -0.2444202 -0.2652648 -0.2331859 

conc.w -0.7317995 -0.0089382 -0.1472482 0.0555924 0.1147823

subs.w -0.0231392 0.0035611 0.0342230 -0.0128427 -0.0297421 -0.0445877
B21

Kit 7b, Lead Chromate

Coefficients:

 Value Std. Error t value 

(Intercept) -1.4121608 0.3423518 -4.1248817 

oper.y1 -0.5237775 0.5245856 -0.9984595

oper.y2 1.0939000 0.5209603 2.0997761 

oper.y3 0.1467141 0.5132734 0.2858401

oper.y4 0.1467141 0.5132734 0.2858401 

conc.y 5.7264090 0.8865833 6.4589631

subs.y -0.5424765 0.2601750 -2.0850445

Null Deviance: 263.1269 on 239 degrees of freedom

Residual Deviance: 99.0'4916 on 233 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.y1 oper.y2 oper.y3 oper.y4 conc.y 

oper.y1 0.1053265

oper.y2 -0.1972383 -0.2746982 

oper.y3 -0.0439092 -0.2615457 -0.2108035

oper.y4 -0.0439092 -0.2615457 -0.2108035 -0.2443067 

conc.y -0.6574002 -0.1454922 0.2738465 0.0666388 0.0666388

subs.y -0.0243484 0.0454522 -0.1031796 -0.0074171 -0.0074171 -0.0999075
B22

Kit 8a, White Lead

Coefficients:

 Value Std. Error t value 

(Intercept) -1.9530873 0.3510377 -5.5637539

oper.w1 -0.2984542 0.3206671 -0.9307289 

oper.w2 0.9278152 0.3141042 2.9538454

conc.w 3.8332350 0.7505190 5.1074453 

subs.w 0.5464517 0.2339381 2.3358817

Null Deviance: 197.8449 on 143 degrees of freedom

Residual Deviance: 123.3189 on 139 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.w1 oper.w2 conc.w 

oper.w1 0.0688350

oper.w2 -0.2826036 -0.4461746 

conc.w -0.7582744 -0.0686413 0.2559359

subs.w -0.3091048 -0.0385199 0.1220282 0.2442332
B23

Kit 8a, Lead Chromate

Coefficients:

 Value Std. Error t value 

(Intercept) -2.10669189 0.4719555 -4.4637507

oper.y1 -0.32967393 0.4086949 -0.8066505 

oper.y2 -0.08313053 0.4058255 -0.2048431

conc.y 4.28701657 0.7412016 5.7838737 

subs.y -0.33922045 0.2878099 -1.1786268

Null Deviance: 181.8984 on 143 degrees of freedom

Residual Deviance: 78.52158 on 139 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.y1 oper.y2 conc.y 

oper.y1 0.0998914

oper.y2 0.0256904 -0.4888234 

conc.y -0.7939533 -0.1286980 -0.0331173

subs.y 0.0462506 0.0280854 0.0094564 -0.0731874
B24

Kit 8b, White Lead

Coefficients:

 Value Std. Error t value 

(Intercept) -2.7048879 0.5711524 -4.7358428

oper.w1 -0.3109273 0.4374891 -0.7107086 

oper.w2 1.5214190 0.4901877 3.1037477

conc.w 15.7459466 3.0870443 5.1006545 

subs.w -0.0901047 0.3065075 -0.2939722

Null Deviance: 189.4789 on 143 degrees of freedom

Residual Deviance: 68.97782 on 139 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.w1 oper.w2 conc.w 

oper.w1 0.1013268

oper.w2 -0.4852529 -0.4427492 

conc.w -0.8442879 -0.1122770 0.5202764

subs.w -0.0834844 0.0176580 -0.0395779 0.0745626
B25

Kit 8b, Lead Chromate

Coefficients:

 Value Std. Error t value 

(Intercept) -2.3320579 0.5822608 -4.0051777 

oper.y1 0.1631747 0.5563792 0.2932796

oper.y2 0.6224517 0.5705703 1.0909290 

conc.y 7.7748472 1.5356701 5.0628370

subs.y -0.1200420 0.3949996 -0.3039040

Null Deviance: 164.1129 on 143 degrees of freedom

Residual Deviance: 45.27508 on 139 degrees of freedom

Correlation of Coefficients:

 (Intercept) oper.y1 oper.y2 conc.y 

oper.y1 -0.0395947

oper.y2 -0.2299963 -0.4519739 

conc.y -0.7423684 0.0763243 0.2912796

subs.y -0.1548276 -0.0059087 -0.0179666 0.1371209