LAUSR

Abstract Background Clozapine is one of the most underused medications in psychiatry for many reasons including mandatory blood testing, fear of serious side-effects, lack of patient adherence. A critical barrier to adoption could be addressed with the ability to measure clozapine at point-of-care (POC) from a fingerstick. Current practice of clozapine measurements, however, was developed based on serum levels. Therefore, meaningful POC results must be reported as the serum equivalent. We evaluated a new immunoassay method to measure clozapine in whole blood to establish standardization to serum, and to assess the ability of the POCT to detect differences in patients’ clozapine levels compared to an existing laboratory method. Methods A whole blood POCT (MyCare® Insite Clozapine Test on the MyCare Insite)* immunoassay was compared to liquid chromatography tandem mass-spectrometry (LC-MS/MS) in serum with 95 matched patient samples. Passing-Bablok regression was used to compare results and establish calibrator values to standardize the POCT to report whole blood results as equivalent to serum. The standardization was validated by a method comparison to LC-MS/MS with 304 samples collected from patients with schizophrenia who were being treated with clozapine. Serial blood levels were taken for 13 patients to compare deviation from baseline for POCT and LC-MS/MS results. To detect a discordant difference in clozapine levels, the difference to the preceding value was calculated for 73 sequential samples of the 86 total results. Because of high intra-patient variability changes of > ±50% were considered significant. Results There was good correlation (R = 0.9) between the POCT and LC-MS/MS in the training set (N=95). Passing-Bablok statistics were: slope = 1.02, intercept = -2.3, R = 0.9, average bias -17.7 (-3.8%). The average values (± SD) were 479.7 (± 181.5) ng/mL for LC-MS/MS and 462.0 (± 199.1) ng/mL for POCT. The Passing-Bablok regression of the validation set (N=304), using the reassigned calibrator values as the training set, gave a slope = 0.971, intercept = -21.2, R = 0.9, mean values (± SD) of 445.6 (± 242.4) for LC-MS/MS and, 412.6 (± 245.7) for the POCT, average bias was -33.0 (-7.7%). Bias between POCT and LC-MS/MS for 12 individuals ranged from -22% to 22%. One patient with five sequential measurements had a total bias of -34% with 4 of 5 results, agreeing with assignment in or out of the presumptive target range of 350 – 600 ng/mL. The frequency of >±50% change in clozapine levels was <5%. Ninety percent (66 of 73) of results agreed, selectivity = 50%, specificity = 94%, positive predictive value (PPV) = 42.9%, negative predictive value (NPV) = 95.5%. Seven samples had a 50% change by one method and not the other. There was only one discrepant sample that was 66% lower with POCT. Discussion Differences in measurement methods are expected. The good correlation and similarity of results between the calibrator assignment training set and the validation set demonstrates the accuracy of the calibrator value assignment. The POCT was highly selective in detecting important changes in clozapine levels of more than 50% which would occur secondary to non-adherence, change in life-style habits or drug-drug interactions. The collection conditions gave consistent levels for most patients, with few large shifts in concentration, thus underestimating the PPV. These data suggest that clozapine levels can be accurately measured from a small volume of capillary blood collected via a fingerstick sample. This method makes blood sampling easier for both patients and clinical staff, and provides a result in a few minutes, at point of care. Its clinical implementation may facilitate the safe and effective use of clozapine in schizophrenia. *CE mark/US RUO