The MRL is defined by the NWQL as the smallest measured concentration of a substance that can be reliably measured by using a given analytical method (Timme, 1995; see Abbreviations and Definitions, p. 19). It is the “less-than” value reported when an analyte either is not detected or is detected at a concentration less than the MRL.
Since the definition of the MRL is not specific, an MRL can be set at any concentration acceptable to the data user and the laboratory as long as reliable measurement is achieved. Examples of specific types of MRL’s include practical MRL’s that are based on regulatory-action limits, water-solubility limits, or analytical capability. Statistically derived MRL’s may include those that are equivalent to a practical-quantitation limit, the limit of quantitation, the method detection limit, the limit of detection, or the limit of identification (Koehn and Zimmerman, 1990).
Historical Use of the Minimum Reporting Level at the National Water Quality Laboratory
The NWQL historically has used MRL’s for reporting non-detections. Establishment of the MRL has been inconsistent across methods and typically inadequately defined and often undocumented. In many cases, statistical procedures may have been used to set MRL’s during development of a method, but the MRL likely was based on subjective criteria (perhaps an easily prepared concentration or the same MRL for all compounds in a method for ease of reporting) or on an analyst’s professional judgment of detection capability. As a result, the NWQL has determined that MRL’s for some methods are no longer appropriate.
Limitations of the Minimum Reporting Level
When MRL’s are set too high, the result is excessive censoring of data below the MRL. The use of higher MRL’s may be useful in applications when analyte concentrations are tested only for exceedance of regulatory criteria, such as drinking-water maximum contaminant levels, or when low-concentration detections are not important. Higher MRL’s increase the probability of providing a true statement that the analyte is present when reported as detected, although it does not necessarily increase the accuracy of the measured value.
When MRL’s are set too low, the result can be data that are misleading. False positives and negatives increase in frequency near the concentration at which the method is unable to distinguish noise from analyte signal. For example, consider two blind samples (meaning an analyst is unaware that they are quality-control samples) that are submitted for analysis using a method where the MRL’s are close to the limit of detection. One sample is a blank and the other sample is a low-concentration spike—spiked at a concentration at or just above the low MRL. Multiple analyses of the blank sample occasionally will result in a detection—a false positive—because the distribution of random signal from the instrument occasionally will result in a concentration high enough to be reported at or above the MRL. Multiple analyses of the low-concentration spike sample will result in an occasional non-detection, even though the analytes are present, because the distribution of measured concentrations occasionally will be less than the MRL—a false negative.
A sample with a true concentration at the MRL, if measured multiple times, would result in a distribution of values—some greater than the MRL and thus reported as detected, and some less than the MRL and thus censored and reported as “<MRL.” Not detected does not imply not present. Data users must be aware that this situation exists regardless of the value of the MRL.
