Clinical issues associated with urine testing of substances of abuse.

Several factors may affect the validity and outcome of urine testing for abused drugs such as amphetamines, barbiturates, benzodiazepines, cannabinoids, cocaine, ethanol, opiates, and phencyclidine. Urine is used for large-scale testing because acquisition of the sample is noninvasive and because most abused drugs can be detected in urine for a reasonable duration after ingestion. Urine testing for drugs of abuse is a two-step process. In the first step, screening assays are used to identify presumably positive specimens. Common screening tests are radioimmunoassays, enzyme immunoassays, fluorescence polarization immunoassay, and thin layer chromatography. Since they may be subject to cross-reactivity, once a possible positive sample has been identified by a preliminary test, a second more specific methodology, gas chromatography with mass spectrometry, is done to confirm the results. Knowledge of the pharmacology and pharmacokinetics of abused drugs affects selection and interpretation of test results.