Urine analysis is primarily used because of its low cost. Urine drug testing is one of the most common testing methods used.
Urine drug tests screen the urine for the presence of a parent drug or its metabolites. The level of drug or its metabolites is not predictive of when the drug was taken or how much the patient used. Rather, it is simply a confirmatory report indicating the presence of the parent drug or its metabolites.
Urine drug testing is an immunoassay based on the principle of competitive binding. Drugs which may be present in the urine specimen compete against their respective drug conjugate for binding sites on their specific antibody. During testing, a urine specimen migrates upward by capillary action. A drug, if present in the urine specimen below its cut-off concentration, will not saturate the binding sites of its specific antibody. The antibody will then react with the drug-protein conjugate and a visible colored line will show up in the test line region of the specific drug strip.