In most clinical laboratories the drug testing progresses in two stages:
- Initial screening with laboratory based immunoassays or point of care (POC) devices
- Confirmation by gas chromatography/mass spectrometry (GC/MS) or liquid chromatography/tandem mass spectrometry (LC/MS/MS )
In the initial screen, classes of drugs or metabolites are assayed and each class is assigned a given cut-off value. A cut-off is a concentration above which an analyte is reported as positive and below which it is reported as negative. This is a qualitative result. Confirmation tests can detect specific drugs and metabolites, rather than classes of drugs, and the results can be reported as positive or negative (qualitative) or an actual concentration can be reported (quantitative result). Because GC/MS or LC/MS/MS instrumentation used in confirmation testing is much more specific and sensitive, the cutoff for a confirmation test can be lower than that of the screening test.
A positive result for a urine drug test is an indication of recent drug use, since most drugs and metabolites are cleared from the urine in one to three days. An exception is the long term use of lipid-soluble drugs, such as marijuana and the antianxiety drug chlorazepate. These drugs can accumulate in fat tissues and be detected in urine for a week or more.
