Implications of drug levels in body fluids: basic concepts

Drug concentrations in biological fluids are affected by the dose, route of administration, pattern of drug use, and the dispositional kinetics (distribution, metabolism, and excretion) of the drug. As most drugs are distributed to the site of action by blood, drug concentration measurement in this body fluid provides the best information as to the potential effect on behavior such as driving impairment or on psychological high. Due to wide individual variations in the pharmacokinetics and pharmacodynamics of drugs, however, the use of plasma drug concentrations for the estimation of impairment has not been established for most drugs. As for urinalysis, drug concentrations in the urine are further complicated by other factors such as urine flow and pH. Even if a specific method is used for the quantitation of a specific drug (the active species, not the inactive metabolite), interpretation in forensic samples to predict time of drug use or impairment is not possible, except within broad time periods, because of the variations in urine drug concentration as well as the limited knowledge available about the dose or the route of administration.