B-293 Comparison of Therapeutic Drug Monitoring for Clozapine by Immunoassay vs. Mass-Spectrometry

Background Therapeutic drug monitoring (TDM) is the clinical practice of measuring the blood concentration of medications to achieve a value within a therapeutic range. Clozapine is a second-generation antipsychotic medication and is considered the most effective first-line therapy for treatment-resistant schizophrenia. The MyCare Psychiatry Clozapine Assay Kit is intended for the in vitro quantitative measurement of clozapine in adult human serum using automated clinical chemistry analyzers. The objective for validating a quantitative clozapine assay was to improve turnaround time to allow real-time dose adjustments if needed and earlier patient discharge from the hospital. During validation, we encountered challenges with the method comparison portion of the validation, and so further investigation was pursued. Methods Samples used in the method comparison validation study included red-top serum samples provided by Saladax Biomedical and spiked in-house red-top serum samples using a 1.0 mg/mL Cerilliant Clozapine stock solution in methanol. Samples were stored at -70°C until testing. Prior to analysis, samples were placed at room temperature to thaw and then mixed via vortex. Eighty-eight samples were compared to three laboratories: Saladax Biomedical (Immunoassay), reference lab A (Immunoassay) and reference lab B (LC-MS/MS). Results Serum samples spiked with a Clozapine stock solution had an overall bias of -50.9% when compared to an LC-MS/MS method at reference laboratory B. In contrast, red-top serum samples from patients taking clozapine compared favorably to laboratories that measure Clozapine by Immunoassay or LC-MS/MS with an overall bias of 5.7% and Deming regression slope of 0.926. Conclusion Following discussion with the vendor, we discovered that the master calibrator levels for the MyCare Psychiatry Calibrator Kit 2 are value assigned to produce assay results that correlate to LC-MS/MS methods for clinical samples. The immunoassay has cross-reactivity with a metabolite of Clozapine; therefore, set points are theoretically based on the Clozapine value minus the expected metabolite cross-reactivity. This allows results of the immunoassay to reproduce results measured by the mass spectrometry method, which only measures the parent Clozapine. Serum samples spiked with a Clozapine stock solution will not recover favorably due to the correction factor for metabolite cross-reactivity not being needed. After excluding the spiked serum samples from the method comparison validation data, we were able to successfully proceed with validation and implementation. The assay has resulted in substantial improvements in turn-around-time and patient care and is greatly appreciated by our clinical colleagues.