In most clinical laboratories drug testing progresses in two stages:
- Initial screening with laboratory immunoassay instruments 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. Each class of drugs is assigned a cut-off value. The cut-off is the concentration above which an analyte is reported as positive, and below which it is reported as negative. A simple positive or negative result is referred to as a 'qualitative' result.
Confirmation tests can detect specific drugs and metabolites, rather than just 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 or the antianxiety drug chlorazepate. These drugs can accumulate in fat tissues and be detected in urine for a week or more.
