Capillary blood sampling for the determination of clozapine concentrations: analytical validation and patient...

Patients may be reluctant to start clozapine (CLZ) because of the frequent blood sampling needed for white blood cell monitoring, but also therapeutic drug monitoring of CLZ. Finger prick, instead of venepuncture, might lower the threshold to CLZ use. The aim of this study was to determine whether CLZ concentrations are the same in capillary and venous blood samples. Capillary blood was drawn by finger prick and collected in the Hem-Col tube. Paired capillary and venous blood samples were collected from inpatients and outpatients of a mental health institute. Patients were asked to rate pain, experience, and preference for blood sampling method. Passing-Bablok analysis of 40 paired samples showed that CLZ concentrations were statistically equal in capillary and venous samples {slope 0.96 [95% confidence interval (CI): 0.88-1.04], intercept -6.86 (95% CI: -30.75 to 24.13)}. Venepuncture was the preferred method based on the experience ( P  = 0.009) and preference ( P  = 0.043) items. In conclusion, CLZ concentrations were similar in venous and capillary blood samples. Venepuncture appeared to be preferred to finger prick for blood sampling. Our results emphasize the importance of consulting patients' individual preference for blood sampling method.