Therapeutic drug monitoring of clozapine and relapse--a retrospective study of routine clinical data.

Objective: Therapeutic drug monitoring (TDM) of the atypical antipsychotic drug clozapine is recommended. Clinical studies have indicated a therapeutic window for clozapine serum levels in schizophrenic and schizo-affective patients during acute treatment, i.e. for patients who do not respond to treatment with typical antipsychotics. However, despite the frequent use of clozapine also in maintenance treatment, very few data are available showing the relationship between serum levels of clozapine and the prevention of relapse. Thus, the primary objective of the study was to investigate the relationship between serum levels of clozapine and relapse during maintenance treatment. Methods: A retrospective study of routine TDM-data was conducted. Samples obtained on an acute treatment ward from patients with 250 ng/ml irrespective of concurrent psychotropic drugs. The risk of intoxication is increased with serum levels > 750 ng/ml. The TDM of clozapine is recommended during maintenance treatment.