The drug screen is a specific assay for a finite number of compounds. Drugs may be detected in urine, plasma, gastric juices, or other body fluids if the volume available is large enough. In general, urine testing is used to screen for the presence of compounds, with confirmation by assaying serum or plasma samples.
The survey is useful as a quick screen for the presence of major categories of drugs of abuse. The Drug Abuse Survey is a urine test for alcohol, amphetamines, barbiturates, benzodiazepines, cocaine, opiates, phencyclidine, and tetrahydrocannabinol (THC) by class, with an immunoassay or thin-layer chromatography procedure. Results are presumptive because no confirmation is done. The false-positive rate for the screen is estimated to be 3% (i.e., of every 100 samples tested, three of them will have false-positive results for one of the drugs in the test group), whereas the false-negative rate for this screen is estimated to be low in the absence of adulterants. Thus, this test has a high true-negative rate. This test requires a random urine sample of at least 30 mL.
This is a urine test for the same drugs covered by the Drug Abuse Survey, but this assay uses gas chromatography—mass spectrometry (GC-MS) to confirm the results of the screening assay. This quantitative test is more accurate and sensitive and more expensive than the Drug Abuse Survey. The GC-MS may be used to test body fluids other than urine. This assay is recommended for employee screening programs or situations in which accuracy is paramount. Specimens should be collected under observation for greater reliability. Some states require that specimens collected for employee screening or forensic purposes be accompanied by documentation showing the chain of custody for each specimen. Substitute urine samples may produce false-negative results, and laboratory mix-ups can produce false-positive results. The GC-MS is considered the standard for testing for drug abuse and has a virtual 0% false-positive rate.