Each parameter in a drug test features a cut-off level. Put simply, this is a threshold, built into the test, at which the result becomes a 'non-negative'. A simple way to think of it is like a speed camera. You need to be travelling at a certain velocity to trigger the camera.
Many cut-off levels are at an internationally agreed level that was originally set by SAMHSA, and they are set at sensible levels, so that the tests will not pick up environmental contamination in a donor, picked up from traces of drugs on surfaces, passive cannabis smoke, etc.
Cut-off levels are set at nanogrammes per millilitre (ng/mL). To give you an idea of the tiny quanities these tests are able to pick up, consider the ratio of a teaspoon of sugar in a swimming pool. If there is drug matabolites in the system, drug tests are highly accurate and sensitive in identifying them.
In a drug test product, there will be a range of cut-off levels across the various drug parameters, and sometimes a test with a lower cut-off level (a more sensitive test) appears to have a fainter 'Test Line' than 'Control Line', or a fainter line than its neighbouring parameter.
Chain of custody is a term given to the process of securing a sample of oral fluid, urine, hair or other substance for transport to the laboratory for either confirmation analysis, or identification analysis.
In all of these situations, it is crucial that the sample isn't tampered with, adulterated or substituted. Adhering to a robust 'chain of custody' procedure, with often includes uniquley identifed and secure packing, ensures that the result from the lab. can be trusted.