title: Plasma quetiapine in relation to prescribed dose and other factors: data from a therapeutic drug monitoring service, 2000-2011.
abstract: OBJECTIVE
  Suggested predose plasma quetiapine target ranges for effective therapy in schizophrenia lie between 50 and 500 µg/l. We aimed to examine data from a quetiapine therapeutic drug monitoring (TDM) service to assess the plasma quetiapine concentrations attained at specified doses in clinical practice.
  
  
  METHODS
  We studied TDM data from patients given immediate-release quetiapine in the period 2000-2011.
  
  
  RESULTS
  There were 946 samples from 487 patients (257 males, age at time of first sample, median [range] 34 [14-87] years, and 230 females, age at time of first sample, median [range] 38 [10-92] years). The plasma quetiapine concentration was <50 and <100 µg/l in 30% and 50% of samples, respectively (no quetiapine detected in 9% of samples). The relationship between dose and plasma quetiapine was poor. The mean (95% confidence interval [CI]) quetiapine dose was higher (t = 3.6, df = 446, p <0.01) in males versus females (641 [600-1240] and 548 [600-943] mg/day, respectively), although there was no difference in median dose (600 mg/day) or in the mean (95% CI) plasma quetiapine concentrations attained. Smoking habit had no discernible effect on plasma quetiapine concentration.
  
  
  CONCLUSIONS
  There was a poor relationship between dose and plasma quetiapine concentration in this study, as found by others. This is probably because of the short plasma half-life of the drug, at least in part. Nevertheless, quetiapine TDM can help assess adherence and measurement of quetiapine metabolites, notably N-desalkylquetiapine, as well as quetiapine itself may enhance the value of quetiapine TDM in future.
authors.affiliation: Toxicology Unit, Department of Clinical Biochemistry, King's College Hospital NHS Foundation Trust, Denmark Hill, London SE5 9RS, UK.
authors.forename: Simon A
authors.initials: SA
authors.lastname: Handley
authors.forename: Sally V J
authors.initials: SV
authors.lastname: Bowskill
authors.forename: Maxine X
authors.initials: MX
authors.lastname: Patel
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Flanagan
keywords: adherence; quetiapine; schizophrenia; therapeutic drug monitoring