Which laboratory test is essential to monitor in a patient taking clozapine (Clozaril)?

• A. BUN and creatinine
• B. White blood cell count
• C. Liver function tests
• D. Brain EEG

Answer: (B)

Clozapine carries a significant risk of agranulocytosis, a dangerous drop in white blood cells that can leave a patient vulnerable to serious infection. Because this adverse effect can develop quietly, regular hematologic monitoring is essential. The most important test to watch is the white blood cell count, typically with the absolute neutrophil count, since maintaining an adequate neutrophil level is what prevents infection. Before starting clozapine, a baseline WBC and ANC are obtained, and then monitoring is integrated into the treatment plan—often with frequent checks early on and then periodic checks if counts stay stable. If the counts fall below established safety thresholds, the medication is held and reevaluated. While other tests like liver function or kidney function can be useful for overall health or drug metabolism, they do not address the specific, time-sensitive safety risk posed by clozapine. An EEG has no role in monitoring clozapine safety.