Abnormal pharmacokinetics: the need for monitoring.

Optimal use of a drug depends on rational dosing and subsequent therapeutic drug monitoring for effectiveness and toxicity. Drug monitoring is not relevant for all drugs, but is indicated in the case of drugs which have a narrow therapeutic range or show a large inter-individual variation. If the response is not satisfactory or toxic side effects are observed, the regimen has to be adjusted or another drug used. Methods have been developed to establish rational dosing schedules for the individual patient. In previous methods, the patient's specific data such as age, length, weight and serum creatinine are integrated with population pharmacokinetic parameters for a drug. This approach is subject to an appreciable margin of error, particularly in patients whose physiology is far from normal. Therapeutic drug monitoring via blood level determination makes it possible to evaluate the patient's individual pharmacokinetic parameters on which a rational dosage regimen can be based.