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Laboratory testing for illegal drugs or alcohol is done for a number of reasons. Most commonly it is done as an employment, insurance, or governmental requirement. While serum (blood) testing is a more accurate test for current impairment and/or time of ingestion, urine is the specimen of choice for most routine screening.

Urine Drug Screening

For routine screening a random specimen is usually collected. Often, there are safeguards to ensure that the specimen is fresh and truly from the patient. In some cases, a strict chain of custody is required.

The substance being tested for or its metabolite often remains in the urine much longer than the impairment or intoxication. This is one reason that urine screening is favored over serum or blood screening.

Interfering Factors

Some common false-positive results in amphetamine testing occur from medications that contain phenylpropanolamine, pseudoephedrine or related drugs. Heavy ingestion of poppy seeds may cause false-positive opiate testing. (Special confirmation testing may eliminate this false-positive.)

False-negative testing may result from overdilution of urine. Other factors causing false-positive or false-negatives also exist.