[Individualization of drug dosage by therapeutic monitoring in plasma and the application of pharmacokinetic principles].

The response to a given drug dose varies widely among patients. This is mainly due to the intra- and interindividual variation in the patients' pharmacokinetics. Individual pharmacokinetic parameters (bio-availability, elimination half-life, apparent volume of distribution, clearance, plasma protein binding) can be calculated from single or multiple dose studies. One should be aware that the age of the patient, his kidney, liver, heart and thyroid function might have a great influence on the pharmacokinetics of a particular drug. The pharmacokinetic parameters can be also estimated for a population of patients by fitting data from routine plasma level monitoring to computer programs, such as NONMEM. For drugs with a narrow therapeutic index, dosage is often based on the measurement of plasma concentrations, which serve as a therapeutic/diagnostic end point. There are various procedures (e.g. nomograms, test doses) for predicting the dosage regimen necessary to achieve so-called therapeutic concentrations in an individual. The general Bayes approach improves forecast precision by combining prior knowledge (population kinetics) with current evidence (measurements of plasma concentrations). Thus, therapeutic drug monitoring and pharmacokinetics can contribute to a more effective and safe therapy.