Individual clearance and therapeutic drug monitoring of quetiapine in clinical practice.

OBJECTIVE: Quetiapine is one of the most frequent prescribed antipsychotics. Based on the consensus guidelines of therapeutic drug monitoring (TDM) in clinical practice, TDM of Quetiapine is "useful". In this pilot study, using a natural sample, we investigated the influence of co-medication, age and gender on the serum concentrations and clearance of Quetiapine. Also we compared the individual clearance in our sample with the expected clearance for healthy subjects, obtained in controlled studies.

METHODS: 150 blood samples were collected anonymously under clinical conditions, Quetiapine trough serum levels were determined using HPLC. Additional information about gender, age, co-medication and dosage was obtained.

RESULTS: There was a significant, positive, but weak correlation between daily dose and serum levels of Quetiapine (r=0.44; p=0.01). When Carbamazepine was co-administered, the clearance was significantly higher (p=0.01). 83% of the serum levels were outside the therapeutic range, only 28% were within the expected clearance.

CONCLUSIONS: In addition to the therapeutic range the individual clearance related to the expected clearance may help to optimize individual pharmacotherapy, especially for combination therapy or in case of incompliance or abnormal metabolism.