How drug screens are performed

The aims of the drug screen are to detect the presence of frequently abused
drugs in the urine of human subjects. Drug screens are used for one of
three purposes:

1) medical purposes (e.g., to monitor a patient's progress in a medical
treatment program for a drug abuse problem the patient has
acknowledged),

2) legal purposes (e.g., to determine if a suspect had taken controlled
substances prior to some accident or crime), and

3) medicolegal purposes (e.g., in an employer's drug abuse program aimed at
both preventing drug-related accidents and crimes and identifying and
treating employees with drug abuse problems).

For medical purposes, laboratories often use simple, less-expensive
methods aimed at identifying specific drugs with which the patient has had
problems in the past. It is not expected that the results of such drug
tests will be used as evidence against the patient in court. If these
results are used as evidence, it is likely that defense testimony will
successfully impugn the evidence.

For legal and medicolegal purposes, more stringent testing is necessary
to obtain information that will successfully withstand technical criticism
in court. Therefore, drug screens done for these purposes often take a
two-tiered approach. First, there is a screening test done on the subject's
urine. This is usually a sensitive test that may have some discrepancies
in specificity (for instance, some popular over-the-counter cold medicines
may yield a positive amphetamine screen). Only if this test is positive for
one or more drugs is the second, more expensive test performed. Generally
courts will uphold testimony based on a drug test if positive results were
obtained on two separate tests based on different chemical methods.