Half-Lives and Windows

One of the more common questions the toxicology laboratory professional is asked by a patient is, "How long will it take before I can pass a drug test." The clinician may ask, "How long should I expect the patient's result to be positive?" The kinetics of drug metabolism and the presence of parent drugs and metabolites in the urine can be hard to predict since urine is not a homeostatically-controlled fluid. Urine concentration, unlike serum concentration, will vary significantly depending on how much one drinks. Also, people metabolize drugs at different rates depending on age, the presence of other drugs, as well as dietary and genetic factors. It takes around five half-lives for a drug to become undetectable. A half-life is the amount of time it takes for a drug concentration in the body to decrease by 50%.