[The determination of drug blood levels: when? which?].

Blood levels of drugs are easy to prescribe but remain difficult to interpret. They are almost useless if pharmacologic effects are measurable. Indications for measurement of blood levels are absence of measurable effects, narrow therapeutic margin, anticipated changes in pharmacokinetics or lack of response. For the interpretation of the results of such determinations a knowledge of at least some basic principles in pharmacology and kinetics is necessary.