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 × 10(9)/l to 19.5 × 10(9)/l, and 1.2 × 10(9)/l to 15.9 × 10(9)/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 × 10(9)/l for WBC, and −0.91 × 10(9)/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 (<3.5 × 10(9)/l) and neutrophils (<1.5 × 10(9)/l) these biases were −1.1 × 10(9)/l for WBC, and −0.25 × 10(9)/l for neutrophils. CONCLUSION: Results from the capillary blood HemoCue WBC DIFF analyser compared well with the venous blood ADVIA 2120i analyser for determining WBC and neutrophil counts. There was a slight overall bias, with the capillary method reporting lower values for both measures. Fingerstick point-of-care analysis is suitable for monitoring blood counts in patients on clozapine, although confirmatory standard venous testing is recommended for test results falling below accepted thresholds.