Clozapine Blood Count Monitoring

In the United States, the Food and Drug Administration requires regular blood count monitoring of all patients taking clozapine. An uncommon but serious side effect of clozapine is severe neutropenia, defined as an absolute neutrophil count (ANC) less than 500. Neutrophils are a type of blood cell that is measured during the blood count monitoring that accompanies clozapine. Neutrophils combat infection. The most dangerous form of neutropenia is agranulocytosis, which occurs in 1% of patients taking clozapine, most commonly between 6 and 18 weeks after starting the medication. Agranulocytosis is usually a sudden drop of neutrophils to a very low level, is thought to be autoimmune, is not permanent, and is a medical emergency. Any severe neutropenia can be fatal if not detected and managed, since it makes people vulnerable to infection. With required monitoring, clozapine is safe, and fatalities from neutropenia have been very rare. To receive clozapine, patients must be part of a system that includes a credentialed clinician, regular neutrophil monitoring, and registry reporting of neutrophil counts. When starting clozapine, blood counts are measured every week for 6 months, then every other week for 6 months, then monthly, if all neutrophil counts are adequate. The full protocol for blood cell monitoring with clozapine is found at

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