Drug testing is the evaluation of urine, blood or another type of biological sample to determine if the subject has been using the drug or drugs in question. There are many circumstances that may require drug testing: Pre-employment drug screening test or random, work-related drug testing to identify on-the-job drug abuse. College or professional athletic drug testing. Post-accident drug testing - a vehicular or on-the-job accident which may have involved human error and resulted in casualties or property damage. Safety-related drug testing - if an employee's job could lead to safety issues if judgement or physical ability were impaired.

A Non-DOT drug test is a drug test given to a worker in an industry that's not regulated by the U.S. Dept of Transportation (DOT).

For the safety of the public, the Department of Transport uses regular DOT drug testing to monitor drug and alcohol use. The DOT is in charge of monitoring drinking and drug use among commercial drivers.

Urine is the most common sample type used for drug testing by employers. A urinalysis will show the presence of a drug in the system after the drug effects have worn off; however, the length of time varies by drug. Urine is the only sample type approved for testing of the federally mandated, safety-sensitive workforce. Typical urine drug tests for employment purposes usually screen for 5 to 10 drugs. Urine screening may detect amphetamines or methamphetamines, barbiturates, benzodiazepines, cocaine, marijuana, MDA-analogues (MDA or MDMA), opiates (codeine, morphine, 6-acetylmorphine [indicative of heroin use], hydromorphone, hydrocodone, oxymorphone, oxycodone), nicotine, or alcohol. Employers may request additional drugs to be screened.

After urine drug screening, oral fluid (saliva) testing is the most common method to test for drug use. It may be referred to as a mouth swab test, and used if an employer or other tester is interested in knowing about recent drug use. It is not ideal to survey long-term use of drugs. Most saliva drug tests can detect usage within a few hours up to 2 days. The donor should avoid any food or beverages for at least 10 minutes prior to the sample being collected. Saliva is an easy lab test to gather samples, is less susceptible to adulteration or substitution, and can be tested for alcohol, barbiturates, benzodiazepines, cocaine, ecstasy, marijuana (THC), opiates, amphetamines, phencyclidine (PCP), and methamphetamines. It is suitable for all testing reasons, including pre-employment testing, random and post-accident testing.

Hair testing may be used to determine drug use over the longer term, usually over a 90-day period of time. Hair can be tested for cocaine, marijuana and THC, cocaine, opiates, amphetamine and methamphetamine, ecstasy, phencyclidine, and alcohol. In general, hair testing allows the longest time frame to detect drugs of abuse. The collector usually takes a 100 gram sample of hair (100 to 120 strands) cut close to the scalp. This method lessens the risk for hair adulteration or substitution, as the sample is collected in full view of the lab personnel.

An applicant is notified that pre-employment drug testing will need to take place as part of the application process. They may have to present to the laboratory within a specified time frame, for example within 24 hours, to lessen the chance that drugs in their system will be excreted and undetectable. Applicants are directed to a specific laboratory to submit a sample for drug screening (usually for urine test results). Once at the facility, the applicant must submit a sample at the discretion of the laboratory personnel and in keeping with their standard policies. Hair, sweat, saliva or blood drug test samples may also be used in pre-employment drug screen, although this is not common practice. During the laboratory evaluation, strict chain-of-custody practices and standards are followed to prevent adulteration of the sample. This legal procedure requires documentation of each person who handles the specimen through the entire phase of testing. Certain laboratory procedures may require direct visual observation while the specimen is being voided, although this is not common. This occurs most often when the donor has previously attempted to tamper with a sample.

A concern for anyone undergoing drug testing is the possibility of a false positive result. Initial screening drugs tests may infrequently result in false positive results, although confirmatory (GC-MS) testing greatly lessens the chances of a false positive - reducing the risk to close to zero. It is important that a person undergoing drug testing complete an accurate history of all prescription, OTC, and herbal drug use prior to the time of the sample collection. Certain substances, over-the-counter (OTC) or prescription drugs may result in false positives due to cross-reactivity with other substances, although many assays have been reformulated to avoid these possibilities. For example, decongestants like ephedrine have been implicated in causing false positives for amphetamines.

Many variables may affect the amount of time that a drug remains detectable in the urine or other biological samples, including: a drug's half-life subject's state of hydration and fluid balance frequency of drug use route of administration cut-off concentration used by the testing lab to detect the drug. General guidelines are available for detection times but can very by individual. Many drugs stay in the system from 2 to 4 days, although chronic use of marijuana can stay in the system for 3 to 4 weeks or even longer after the last use. Drugs with a long half-life, such as diazepam, may also stay in the system for a prolonged period of time.