Drug Screening

Laboratory testing for illegal drugs or alcohol is done for a number of reasons. According to the American Management Association, approximately 76 percent of employers require pre-employment drug testing. While serum or blood testing is a more accurate test for current impairment and time of ingestion, urine is the specimen of choice for most routine 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. Some common drug screening tests along with their "maximum detection times" are listed in the table below.

Some 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-positives or false-negatives also exist.