ABSTRACT
One of the most fundamental assumptions in pharmacology is that drug response at a given time is determined by the quantity of drug attached to the sites of action. Measurement of drug levels in blood is based on the understanding that there are no significant variations with respect to the pharmacological effects produced by certain well-defined blood concentrations of most drugs. Therapeutic levels of most drugs are usually in the submicrogram range, and development of sensitive and reproducible techniques, accessible to the clinical laboratory, is a necessary prerequisite for the accurate monitoring of drug levels in the human body. Dosages required to produce the desired therapeutic effect and which are tolerated without producing toxic effects differ widely among individual patients. The concept of bioavailability has proved useful in distinguishing between different drug formulations. Gas liquid chromatography is the most widely used technique at present for the quantitation of the levels of drugs and their metabolites in biological fluids.
