Pitfalls and errors in drug monitoring: pharmacokinetic aspects.

The measurement of drug concentrations in biologic fluid has become an imperative part of drug research, clinical studies and drug monitoring in therapy. The use of blood "levels" often results in non-conclusive information, misinterpretation and failure. Besides logistic, compliance and analytical problems pharmacokinetic aspects also ought to be considered for reliable use of drug concentrations. The following pharmacokinetic aspects are discussed: number of blood samples, optimal timing for blood sampling, exact recording of time of dose given, exact recording of sampling time, potency, dosage form used, circadian rhythm, and steady state versus non-steady state.