A novel approach to quantitative LC-MS/MS: therapeutic drug monitoring of clozapine and norclozapine using isotopic internal calibration

Therapeutic drug monitoring (TDM) requires timely results in order to be clinically helpful. Such assays, when carried out using mass spectrometry-based methods, typically involve a batched sample approach with multipoint calibration. Isotopic internal calibration offers the possibility of open-access mass spectrometric analysis with consequent shortening of turnaround times. We measured plasma clozapine and N -desmethylclozapine (norclozapine) concentrations in (1) external quality assessment (EQA) samples ( N = 22) and (2) patient samples ( N = 100) using liquid chromatography-tandem mass spectrometry with isotopic internal calibration (ICAL-LC-MS/MS). Analyte concentrations were calculated from graphs of the response of three internal calibrators (clozapine-D 4 , norclozapine-D 8 , and clozapine-D 8 ) against concentration. Precision (% RSD) and accuracy (% nominal concentrations) for the ICAL-LC-MS/MS method were