Drug testing is done primarily to screen people systematically or randomly for evidence of use of one or more substances with potential for abuse. Testing is done in the following circumstances:
Notification or consent may be a requirement before testing, depending on jurisdiction and circumstances. Mere documentation of use may be sufficient for legal purposes; however, testing cannot determine frequency and intensity of substance use and thus cannot distinguish casual users from those with more serious problems. Also, drug testing targets only a limited number of substances and thus does not identify many others. The clinician must use other measures (eg, thorough history, questionnaires) to identify the degree to which substance use has affected each patient's life.
Alcohol, marijuana, cocaine, natural and semisynthetic opioids, amphetamines, and phencyclidine are the substances most commonly tested for. Testing for benzodiazepines and barbiturates may also be done. Urine, blood, breath, saliva, sweat, or hair samples may be used. Urine testing is most common because it is noninvasive, quick, and able to qualitatively detect a wide range of drugs. The window of detection depends on the frequency and amount of drug intake but is about 1 to 4 days for most drugs. Because cannabinoid metabolites persist, urine tests for marijuana can remain positive longer after use is stopped. Blood testing can be used to quantify levels of certain drugs but is less commonly done because it is invasive and the window of detection for many drugs is much shorter, often only hours. Hair analysis is not as widely available but provides the longest window of detection, ≥ 100 days for some drugs.
Validity of testing depends on the type of test done. Screening tests are typically rapid qualitative urine immunoassays. Such screening tests are associated with a number of false-positive and false-negative results, and they do not detect meperidine and fentanyl. Lysergic acid diethylamide (LSD), gamma hydroxybutyrate (GHB), mescaline, and inhaled hydrocarbons are not detected on readily available screens. Confirmatory tests, which may require several hours, typically use gas chromatography or mass spectroscopy.
Several factors can produce false-negative results, particularly in urine testing. Patients may submit samples provided by others (presumably drug-free). This possibility can be eliminated by directly observing sample collection and by sealing samples immediately with tamper-evident seals. Some people attempt to defeat urine drug testing by drinking large quantities of fluids or by taking diuretics before the test; however, samples that appear too clear can be rejected if specific gravity of the sample is very low.
False positives can result from ingesting prescription and nonprescription therapeutic drugs and from consuming certain foods. Poppy seeds may produce false-positive results for opioids. Pseudoephedrine, tricyclic antidepressants, and quetiapine may produce false-positive results for amphetamines, and ibuprofen may produce false-positive results for marijuana.
Last full review/revision July 2008 by Patrick G. O'Connor, MD, MPH