Long Term-Method Detection Levels (LT-MDL)
for the US Geological Survey, National Water Quality Laboratory

Definitions

A long-term method detection limit (LT-MDL) is a modification and enhancement of the EPA 40CFR Part 136 defintion of the method detection limit (MDL). It includes more data points, includes a more thorough capture of laboratory variability, and is calculated using three different possible data assessments. An LT-MDL is a censoring limit for most analytical methods at the US Geological Survey's National Water Quality Laboratory. The purpose of the LT-MDL is to limit the false positive rate to less-than or equal to one percent.

A Laboratory Reporting Limit (LRL) is the less-than value reported to customers when nothing is detected. The LRL is set at twice the LT-MDL. The LRL is set to limit the false negative rate to less than or equal to one percent. The use of the LT-MDL and LRL require the use of a specific reporting scheme.

An LT-MDL, when determined using low-concentration spiked samples, assumes that the spike distribution is equivalent to the blank distribution. The distribution of the spikes is re-centered on zero, as if it were derived from blanks. The LT-MDL is then determined at one percent on the right-tail of the distribution.

LT-MDL = s x t_{(n-1, 1-α = 0.99)}

The LRL is set at two times the LT-MDL so assure that the false negative rate is equal to or less than one percent.

Alternatively, the LT-MDL can be set based on the 99th percentile of the blank population.

REFERENCES:

USGS Series	Open-File Report
Report Number	99-193
Title	New reporting procedures based on long-term method detection levels and some considerations for interpretations of water-quality data provided by the U.S. Geological Survey National Water Quality Laboratory
Edition	-
Language	ENGLISH
Author(s)	Childress, Carolyn J. Oblinger; Foreman, William T.; Connor, Brooke F.; Maloney, Thomas J.
Year	1999
Originating office
USGS Library Call Number	(200) R29o no.99-193
Physical description	iv, 19 p. ill. ;28 cm.
ISBN