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 may be done in the following:
Certain groups of people, commonly including students, athletes, and prisoners
People who are applying for or who already hold certain types of jobs (eg, pilots, commercial truck drivers)
People who have been involved in motor vehicle or boating accidents or accidents at work
People who have attempted suicide by unclear means
People in a court-ordered treatment program or with terms of probation or parole requiring abstinence (to monitor adherence)
People in a substance abuse treatment program (as a standard feature, to obtain objective evidence about substance abuse and thus optimize treatment)
People required to participate in a drug testing program as part of custody or parental rights
Members of the military
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 volume of substance use and thus cannot distinguish casual users from those with more habitual use. Also, drug testing targets only a limited number of substances and thus does not identify many others. The clinician must use other measures (eg, history, questionnaires) to identify the degree to which substance use has affected each patient’s health and functioning.
The substances most commonly tested for are
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.
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.
False results
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
