[Monitoring of therapy using drug concentration levels: Why? When? How?].

Serum concentration measurements represent a tool that can help the physician to choose the optimal dose in an individual patient. Its application has been recommended for several drugs that show a narrow therapeutic range and for which the dose cannot be individually adjusted directly on the basis of the therapeutic effect. For an effective use of his therapeutic tool, it is important to observe the following three points: 1. The dosage history must be known, including patient's compliance. 2. Whenever possible, the measurement should be obtained after steady-state has been reached on a constant dosing regimen (four half-lives of the drug). The dose adjustment in steady-state is very simple: The concentration changes are for most drugs proportional to the changes in dose rate. 3. The dose adjustment should never be performed on the basis of the concentration measurement alone, but always in the context of the clinical conditions and the patient's response.