Every instrument has a concentration below which it cannot distinguish a substance from background noise.
Every instrument has a concentration below which it cannot distinguish a substance from background noise. A report saying not detected means below that threshold, which is a statement about the instrument as much as about the sample.
The limit of detection is the lowest concentration that can be reliably distinguished from nothing. The limit of quantification is higher: the lowest concentration that can be measured with a stated accuracy. A result between the two is real but not reliably numbered.
Without the threshold, not detected is uninterpretable. A sensitive method finding nothing is informative; an insensitive one finding nothing is close to meaningless, and the report looks identical in both cases unless the limit is stated.
Thresholds depend on instrument, method and sample preparation. Two laboratories can honestly report different findings on identical material because one could see further down than the other, and neither is wrong.
A figure reported just above the quantification limit carries large relative uncertainty. Treating it as a precise measurement overstates what was established, and comparing two such figures between labs compounds the problem.
Tests for things that should be absent are exactly where the threshold matters most, because the entire result is a statement about what could not be seen. A contamination result without a stated limit is the weakest common form of published lab evidence.
The limits are established during validation, by measuring known low concentrations repeatedly. A laboratory stating its limits is implicitly stating that it validated the method, which is a meaningful claim about its practice.
A stated limit beside any not-detected result, the units it is expressed in, and whether the limit is detection or quantification. Reports giving all three are unusual and should be weighted accordingly.
The claims on this page about not found means below a threshold, and the threshold is rarely stated can be checked against sources outside this site: