Making the Most of Hating the MDA Good Practices for Using Detection Limits and Decision Levels

1. Making the Most of Hating the MDAGood Practices for Using Detection Limits and Decision Levels Keith McCroan, Bob Shannon, Stan Morton, Doug Van Cleef September 2010 For the ASP 2010 Workshop

2. Agenda: Quick Review of the Quest What’s wrong with the MDA? What do we do without it? We’ve tried this before, why again? Some ways to do this right 2

3. Reminder of the basis (what’s wrong with the MDA): Pretty much everyone uses some form of ‘MDA’ (MDC, LLD, DL, etc.) The MDA is widely misapplied We began with: Clear understanding of the roots of the MDA Proper methodologies for deciding if an analyte is ‘present’ How to properly use the MDA Alternatives to the MDA 3

4. A little background reminder: L.A. Currie, 1968, introduced the LD (detection limit) in a paper titled “Limits for qualitative detection and quantitative determination; application to radiochemistry.” This concept has been expanded on and expounded on many times and now includes the minimum detectable activity (MDA) and minimum detectable concentration (MDC) concepts Currie has ‘clarified’ the detection limit and MDA usage concepts repeatedly 4

5. In 1995, Currie wrote: “the single, most important application of the detection limit is for planning (CMP(1) design or selection). It allows one to judge whether the CMP under consideration is adequate for the detection requirements. This is in sharp contrast to the application of the critical value for decision making, given the result of a measurement. The most serious pitfall is inadequate attention to the magnitude and variability of the overall blank(2), which may lead to severe underestimation of the detection limit.” (3) Chemical Measurement Process Or suitable analyte-free reference, e.g. spectrum continuum L.A. Currie: Nomenclature in evaluation of analytical methods including detection and quantification capabilities PAC 67 (10) 1699-1723 (1995) 5

6. So, …. Stop using the MDA to treat results. 6

7. How? Consider: MQC instead of MDA Better understanding of MQO Use the MDA in the manner intended Critical Level not MDA ASTM D19.04 trying to help MARLAP treatment of detection decisions Different Detection or Acceptance Criteria 7

8. For example - an analyte is detected in the environment suggesting action must be taken. Two questions the project must address involve specifying the level of analytical effort needed, and deciding whether a measurement indicates the presence or absence of analyte with a specified high degree of confidence.  The “how hard should we look” question is best answered by looking at the a priori MDA, and the “is it present” question is best answered by comparing results to the a posteriori decision value. 8

9. Establish the tolerance for making decision errors – for example, 5% Stay away away from making detection decisions using the MDA – it should be done using the critical level.  9

10. For example - an action level has been established for analytes.  The MARLAP approach of specifying the Action Level, DQOs and MQOs (e.g., Required Method Uncertainty) provides not only statistically defensible data, but it can also save a lot of money, time and effort by ensuring that the level of analytical effort is appropriate to produce data of sufficient quality to make decisions. 10

11. Depending on the action level and the tolerance for decision errors, this might mean that more effort is needed. Sometimes this might mean that less effort is needed, which can lead to significantly lower analytical costs. 11

12. Detect (or action) decisions: 1. Critical Level (is it there?) 2. Use the upper bound of the measured activity and uncertainty (does it matter?) 12

13. Critical Value

14. How to get a better detection decision: Projects must take the DQO process seriously - including a serious evaluation of decision errors Projects must translate DQOs into MQOs for the lab and take the recommendations of MARLAP to heart Projects must also consider the sampling uncertainty when looking at establishing the required method uncertainty. 14

15. Labs need to become familiar with MQOs such as the required method uncertainty and the minimum quantifiable concentration Labs must stop assessing their QC and qualifying their results by comparing to the MDA – THIS IS TECHNICALLY WRONG!!!! Labs should compare results to the critical level to decide whether analyte is present or not  (Should be doing this anyway!) 15

16. Labs should take the recommendations of MARLAP to heart – better data will result.    16

17. In summary: MDA is an a priori tool for evaluating the usefulness of methods MDA has no routine place in a posteriori result evaluations Decision Level and Uncertainty can be used to determine if an analyte is present or if it matters MARLAP offers a common, comprehensive, industry-standard approach to decision methodology We can do this better! 17