A retrospective study of clozapine and norclozapine concentration in patients with schizophrenia: Data from the Therapeutic Drug Monitoring Service, 2019–2022

The aim of this research was to assess the therapeutic drug monitoring (TDM) of clozapine (CLO) and norclozapine (NCLO). TDM results of CLO and NCLO in patients obtained from the Xi'an Mental Health Center were retrospectively analyzed. TDM of CLO and NCLO was typically conducted only once in the majority of patients, particularly those receiving outpatient care. The CLO plasma concentrations were higher in inpatients and female patients. The interquartile (25th–75th) CLO concentrations ranged from 129.83 to 397.53 ng/mL, nearly 68.63% of the samples had subtherapeutic concentrations (<350 ng/mL). Receiver operating characteristic curve analysis showed that inpatients achieved the therapeutic level concentration of 350–600 ng/mL when their daily CLO dose was > 125 mg. It was surprising to find such a large number of patients with CLO levels below the therapeutic range, there is still a window of improvement for optimizing pharmacological treatments in clinical practice.