Screening Methods

All SAMHSA-mandated testing is performed in two steps: immunoassay screening followed by gas chromatographic confirmation. For some drugs, cutoff concentrations differ between steps. All samples must be submitted and tested with strict chain-of-custody documentation.

Specimens. The preferred specimen for drug testing now is urine, although alternative specimens such as hair, oral fluids (saliva), and sweat have potential (see Table I). The half-life of drugs in blood is short, making blood specimens less useful for routine drug screening (though a high concentration of active drug will suggest recent usage and, sometimes, likelihood of impairment).

Hair, if long enough, can be a record of drug use over weeks and even months. No convenient screening procedures for the measurement of drugs in hair exist, and hair tests lack the sensitivity of urine tests for occasional or single-time drug use.

Oral fluid and sweat samples are less prone to adulteration than urine but, because drug concentrations are lower in oral and sweat samples, they require more-sensitive test methods. Sweat patches are being considered for use in drug-compliance programs. Applied with tamper-evident tape, and later removed by the program monitor for laboratory analysis, a sweat patch could track an individual over several days. Collecting samples large enough for confirmation testing and retesting would be difficult, however.

Urine specimens are preferred for testing because large sample volumes can be collected noninvasively. Drugs generally remain detectable in urine for two to three days, longer than in blood. THC can remain positive in urine for several weeks after the last use, especially in chronic users. A positive THC urine drug test does not necessarily imply impairment of the donor, however, because THC is usually inactivated by the liver within a few minutes or hours after administration.