February 6, 2014 

Mr. Lemuel Walker 

U.S. Environmental Protection Agency 

Clean Water Act ATP Coordinator 

Engineering and Analysis Division 

Office of Science and Technology 

Office of Water [4303T] 

1200 Pennsylvania Avenue, N.W. 

Washington, D.C. 20460 

Regarding: Recommendations regarding the proposed update to the 40 CFR Part 136 method detection limit 

procedure

Dear Mr. Walker, 

The Environmental Laboratory Advisory Board (ELAB or Board) is a standing Federal Advisory Committee Act 

board that advises the U.S. Environmental Protection Agency (EPA or Agency). The Board’s Charter states 

that it is to provide consensus advice, information and recommendations on issues related to EPA 

measurement programs and facilitate operation and expansion of a national environmental laboratory 

accreditation program. 

ELAB has reviewed the proposed update to the method detection limit (MDL) drafted by The NELAC Institute 

(TNI) Chemistry Expert Committee. In ELAB’s opinion, the proposed update is a considerable improvement to 

the current MDL procedure, and therefore the Board recommends that EPA adopt the proposed changes as 

soon as reasonably possible. 

The Board recommends a few minor changes to the current draft procedure that ELAB believes improve clarity 

or strengthen the procedure. However, these changes do not alter the overall intent, and ELAB would 

recommend adoption even without incorporation of the suggested changes. 

Thank you for your consideration of the Board’s comments regarding TNI’s proposed revisions to the MDL 

procedure. 

Respectfully, 

Patsy Root 

Chair, Environmental Laboratory Advisory Board 

cc: ELAB Board

 TNI Chemistry Expert Committee 
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ELAB Comments on the Revised Method Detection Limit (MDL) Procedure 

Background 

The detection limit of an analytical procedure is a critical property and vital to understanding the capability of 

the method and the range of data quality objectives that can be supported. The MDL serves the purpose of 

determining the single laboratory detection limit for methods approved under 40 CFR 136, and the importance 

of accurate estimates can hardly be overstated because incorrect compliance decisions are likely to result from 

an inaccurate estimate. Unfortunately, the current MDL procedure is widely recognized to have serious flaws 

that can and do result in incorrect detection limit estimates. 

There have been previous efforts to update or replace the MDL procedure. In 2003, EPA issued a proposed 

update that was withdrawn after receiving considerable negative comment in the public review period. 

Subsequently, a federal advisory committee developed a replacement for the MDL that was generally considered technically sound, but implementation difficulties were such that it was never implemented. 

In the Board’s opinion, the TNI procedure strikes a good balance between technical validity and ease of 

implementation. Detailed comments are provided below, and suggestions for language changes are included 

marked as revisions in the attached copy of the procedure. 

Definition: ELAB supports the change in definition to reference method blanks rather than zero. For the 

purposes of determining compliance, it is important to identify the lowest level that can be reliably distinguished 

from a blank rather than zero. 

Section 2.a Selection of spiking level: Changing the initial spiking level from 1 to 5 times the estimated MDL to 

2 to 10 times is beneficial. Spiking at only 1x the MDL is likely to result in many results below the MDL (i.e., the 

spiking level would be too low). It may be helpful to point out that spiking levels 10x the anticipated MDL should 

only be needed if the specific analyte is a poor performer in terms of recovery. 

Section 2.b Blanks: The Board strongly supports including an assessment of blanks in the MDL procedure. As 

the objective is to identify the lowest level that can be distinguished from a blank, using actual blank data 

makes sense. 

Section 2.b.i Multiple instruments: The guidance for multiple instruments is valuable. This is a very common 

situation and lack of previous guidance has resulted in a wide and confusing range of different approaches. 

Section 2.c.iii Computation of the MDL based on blanks: ELAB supports calculation of the MDL based on 

blanks as mean plus Student’s t times the standard deviation. Incorporation of the mean is very important. 

Section 2.d Set the MDL: The Board believes that EPA should consider favoring the blank-calculated MDL 

over the spike-calculated MDL, assuming that there is more blank data than spike data. 

Section 3. Ongoing data collection: ELAB believes that spreading data collection over time is important, and 

the requirement for a minimum of two spikes per quarter is a reasonable compromise between collecting 

sufficient data and having a procedure that is not too onerous. EPA may want to consider providing clearer 

guidance regarding what is meant by “per quarter.” 

Section 4. Ongoing annual verification: Although an annual recalculation of the MDL is not in the current 

procedure, the Board believes that this recalculation (annual verification) is a sound practice for maintaining 

MDLs that reflect the current capability of the laboratory; therefore, ELAB believes that it is a good addition to 
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the procedure. EPA may wish to consider clarifying the annual requirement. For example, “recalculation of the 

MDL must be performed within 13 months of the previous MDL determination or recalculation.” 

Section 4.f. Adjustment of the MDL: No justification is provided for the factor of three used as a limit for 

determining whether the current MDL needs to be adjusted. Some degree of justification would be useful and 

ELAB notes that the upper 99% confidence interval for a population standard deviation based on 6 degrees of 

freedom is 2.98. 

Addressing Previous Objections to the MDL 

ELAB has reviewed objections to the 2003 MDL update that were received in the public comments, with a view 

to determining whether they have been addressed in the new draft. Many comments were received on each of 

the following general issues. 

Variability: 

Long-Term Variability—The new draft does a good job of incorporating longer term variability. 

Interlaboratory Variability—There is no additional material covering interlaboratory variability; however, 

the procedure is intended as a single laboratory detection limit determination. Recently, EPA has set 

MDLs based on the highest MDL from participating laboratories, after removal of outliers, and the same 

approach could be used with the new draft. Another option for estimating the interlaboratory variability 

would be to calculate a pooled MDL from a large population of laboratories. ELAB would be able to 

assist EPA in gathering this data once the revised MDL has been in use for a year or more. 

Analyte Concentration, Calibration and Analytical Range—Determining the spiking concentration is 

similar in the current and new drafts, but the new draft has the benefit that there are steps to verify the 

MDL determined. 

Definitions: Many concerns were raised regarding the definition of the MDL. ELAB believes that adjusting the 

definition to refer to blanks, and clarifying that the MDL is based on results rather than true concentrations, will 

be beneficial in removing some of the current confusion. The MDL does not purport to be equivalent to the 

IUPAC limit of detection, but is more similar to the critical level because it only controls false positives. 

Precision and bias: Concerns were raised especially concerning bias, and the new draft addresses these well 

by incorporating the mean of the blanks into the MDLb calculation. 

Error types: The current MDL does nothing to control false negatives. The revised draft addresses false 

negatives to some extent through the requirement that the ongoing spikes return positive results. The Board 

would encourage EPA to consider further control of false negatives through a subsequent effort to provide 

better definition of a single laboratory quantitation limit, which could be used to replace the minimum level. 

Iterative procedure: There were many complaints regarding the proposed iterative procedure in the 2003 

proposed update. This iterative procedure has fortunately been removed from the current draft and replaced 

with ongoing evaluations of the MDL. 

Outlier testing: There were various comments both in favor and opposed to outlier testing. The new draft does 

not include an outlier test, but the nonparametric option for method blanks could be considered a useful 

alternative. 
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Spike levels: The new draft includes options and requirements for adjusting spike levels if needed based on 

the ongoing data collection. 

Use of blanks: Many comments concerned the lack of consideration of blanks in the 2003 draft; these are fully 

addressed in the new draft. 

Number of replicates: Some comments noted that more replicates would be desirable. The new draft still 

allows starting with seven replicates (a reasonable compromise), but in most cases many more replicates 

would soon be available through the ongoing data collection. 

Tolerance vs. confidence intervals: Some comments stated that tolerance intervals should be used for the 

MDL. This is a point of contention between statisticians, and ELAB does not take a position on which is 

preferable. However, retaining Student’s t will certainly make implementation of the new draft easier, and in 

many cases, the increased number of replicates will reduce the difference between the confidence and 

tolerance intervals. Also, the use of nonparametric statistical tools when possible eliminates this contentious 

issue. 

Sensitivity check: Comments in favor of a sensitivity check are addressed by the requirement for ongoing data 

collection and the requirement that the spikes return positive detections.