Using a fingerstick test for haematological monitoring in patients treated with clozapine

Background Treatment with clozapine requires regular blood monitoring in order to minimise the risk of agranulocytosis. The demands on patients and clinicians associated with monitoring may be reduced by using point-of-care, as opposed to lab-based assessments. We assessed the utility of a device that can measure white blood cell (WBC) and neutrophil counts by capillary fingerstick blood. Method The performance of a small, portable device (HemoCue® WBC DIFF System) was compared with that of a widely used laboratory analyser (ADVIA® 2120i) for measuring WBC and neutrophil counts. Patients with schizophrenia who were being treated with clozapine (n = 201) provided a fingerstick capillary sample and a venous sample for the respective assays. Results WBC counts and neutrophil counts from venous blood as determined by ADVIA 2120i, ranged from 3.0 × 109/l to 19.5 × 109/l, and 1.2 × 109/l to 15.9 × 109/l, respectively. There was a strong correlation between the results from venous and the capillary sample methods (WBC: R = 0.89, neutrophil: R = 0.92). By Passing–Bablok regression analysis, the slope of the association between ADVIA® 2120i and HemoCue WBC DIFF for WBC was 1.0 [95% confidence interval (CI) 0.944–1.086], with intercept at −0.9 (95% CI −1.43 to −0.45). For neutrophils, the slope was 0.870 (95% CI 0.817–0.923), with intercept at −0.19 (95% CI −0.43 to 0.02). Overall, mean biases of −0.95 × 109/l for WBC, and −0.91 × 109/l for neutrophils were observed for the capillary blood method compared with the venous blood method. Below the clinical cutoff intervals for clozapine monitoring WBC (