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Standard: 2003 NELAC
Section: Appendix D.1.2.1 (c)
Link to relevant standard
REQUEST:
Our laboratory was recently cited with "The laboratory has not established procedures to relate LOD's with Limits of Quantitation (LOQ's) (EPA 200.8, 300.0, 6020, 9056)."
I must be missing something here, or perhaps an expectation is being created by this standard, which is a favorite deficiency for auditors.
The LOD (which is not clearly defined in the NELAC standard) is interpreted by our company to be equivalent to the MDL. The MDL is derived from an MDL study following the guidance in 40CFR136.
The LOQ is interpreted by our company to be lowest concentration that can be reliably and accurately reported, and it is required that it be encompassed in the calibration curve. Typically the low point of the calibration curve is equal to the LOQ when adjusted for sample preparation factors.
In this context, the LOD and the LOQ are numbers that are derived independently, and the only requirement is that the LOD be equal to or less than the LOQ. However, auditors are implying that there should be a fixed factor (or range, such as 1-10) separating the LOD and the LOQ. In our laboratory, metals by ICP-MS have ratios of LOQ/LOD that range from 1.4 to 100. In my 20+ years in environmental laboratories, I have seen LOQ/LOD ratios that are all over the place.
Is the standard giving me permission to adjust MDLs upward, provided that they are supported by MDL studies, so that the LOD is withing a fixed factor of the LOQ? This would be convenient, because the LOD would not change from year to year. Our clients would be very appreciative of invariable LODs. The question is, is this something that NELAC supports?
I would like to see some clarification on this standard, because the way it is written, it seems to create the perception that we are missing something.
TNI FINAL RESPONSE:
Quality Systems Expert Committee/NELAP Board, 10-x-08: Since the lab is using the low standard as their LOQ, and the 40CFR value as their LOD, there won’t be a mathematical ratio between the two. If the lab can point to a statement in their SOPs or Quality Manual that the LOQ is greater than the LOD, they would be in compliance.