(Originally presented August 25, 2011)

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DISCLAIMER

This information is distributed solely for the 

purpose of pre-dissemination peer review under applicable information quality 

guidelines. It has not been formally 

disseminated by EPA. It does not represent 

and should not be construed to represent any 

Agency Determination or Policy.

The information provided should not be cited 

or referenced.

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Presentation Overview

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 Overall goal 

 Available data

 Method performance and product performance

 2009 collaborative study design

 The data and use of the MPN formula

 Identifying sources of variability

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Overall Goals of the Reassessment

 Short-term/mid-term solution while we implement OECD 

method 

 Use the best data available and apply the best available 

statistics

 Include input from stakeholders, i.e., the EWG

 Seek independent technical assessment, e.g., AOAC

 Incorporate critical UDM revisions 

 If approved, provide detailed communication and 

instructions to end-users 

 Ensure long term monitoring process

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Available Data

Data available for analysis from recent 5-lab 

collaborative study with industry

 Originally designed with industry to assess 

method modifications, proposed to resolve the 

pellicle issue.

 MLB was the lead lab; 1 contract lab and three 

industry labs

 The modifications were shown not to improve 

the UDM; however, the current UDM exhibited 

acceptable performance overall. 

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Contrasting Method Performance and 

Product Performance

 Method Performance

 Method’s ability to generate 

consistent and reproducible 

data under typical lab 

situations

 Enhanced by following 

standard methods and SOPs

 Variability between UDM 

tests within a lab and 

between labs, of the same 

product

 Control counts

 Responsiveness to changes in 

product effectiveness

 Product Performance

 Efficacy – using qualitative 

and quantitative tests

 Per formulation type

 Per label claims and test 

conditions

 Neutralization critical to 

measuring efficacy 

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Microbe Day 1 (Rep #1) Day 2 (Rep #2) Day 3 (Rep #3)

S. aureus Test 1: Current UDM Test 1: Current UDM Test 1: Current UDM

Test 2: Modified UDM Test 2: Modified UDM Test 2: Modified UDM

P. aeruginosa

Day 4 (Rep #1) Day 5 (Rep #2) Day 6 (Rep #3)

Test 1: Current UDM Test 1: Current UDM Test 1: Current UDM

Test 2: Modified UDM Test 2: Modified UDM Test 2: Modified UDM

•Use of MLB’s SOP

•One QAC product with the label claim

•Five highly representative labs

•60-carrier tests 

•6 log minimum carrier count

•A total of 30 tests utilized in the reassessment

•Order of microbe randomized 

Basic Study Design for the 2009 

Use-dilution method Collaborative

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Data from the 2009 collaborative

A. Numbers of positive carriers out of 60

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Data from the 2009 Collaborative

B. Control carrier log densities (mean +/- 1 SEM)

For Pa, SEM = 0.14; for Sa, SEM = 0.12.

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Carrier counts

in combination 

with the 

number of 

positives 

log reduction 

(LR)

For statistical analysis, conversion of number of positives 

to log reduction values – use of the MPN formula

Use of the 

MPN formula 

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Use of the MPN formula

 Widely used by FDA – see Blodgett, R.J., (2006) Most 

Probable Number Determination from Serial 

Dilutions. FDA’s Bacteriological Analytical Manual 

 Used by EPA in the past – see Tomasino and Hamilton 

(2006). Modification to the AOAC Sporicidal Activity 

of Disinfectants Test: Collaborative Study. JAOAC Int. 

89:1373-1397

 See MSU’s “Knowledge Sharing Article” KSA-SM-08 

for complete details on the P/N formula at 

http://www.biofilm.montana.edu/content/ksa-sm-08 

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within-lab variability

Identifying sources of variability

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mean LR is 8.26

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Among-lab variability

Identifying sources of variability

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Within-lab variability

Among-lab variability

Reproducibility SD = 0.5348 

Identifying sources of variability

Using an assessment 

variability, we can develop 

the probability of events 

through modeling 

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For 60 carrier UDM tests, P/N LR values are displayed for each 

combination of the observed number of positive carriers

16

0 10 20 30 40 50 60

8.5

8.0

7.5

7.0

6.5

6.0

No. Positive

P/N

L

R

P/N LR for 60 carrier UDM when the mean control LD = 6.5

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Important Questions to Ask

 Type 1 Error

 How often does a 

marginally ineffective 

product pass?

 Example: using the 

current performance 

standard of allowing 0-1 

positive

 Example: using a new

performance standard 

 Type 2 Error

 How often does a highly 

effective product fail?

 Example: using the 

current performance 

standard

 Example: using a new 

performance standard

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Overall Recommendations

Increased 

confidence 

in the 

outcome

Revise 

performance 

standards for 

each microbe

Require 

multiple testing 

of the same 

product Use other 

enhancements 

to the UDM 

such as max. 

carrier counts

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Additional technical review

 Manuscript for JAOAC describing procedural revisions 

and the proposed performance standards under 

preparation

 Submit to JAOAC

 Independent panel of experts is assigned for review

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