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The use of clozapine is limited by the risk of agranulocytosis. The incidence of agranulocytosis after 1 year was .80 percent in 11,555 patients registered in the Clozaril Patient Management System (CPMS) who received clozapine from February 1990 to April 1991. We noticed a tendency for white-cell counts to spike upward before agranulocytosis occurred. We analyzed the CPMS data to test whether a white-cell count spike at least 15 percent above the previous measurement predicted agranulocytosis within 75 days, with a 21-day lag to allow white-cell counts to decline to levels indicative of agranulocytosis. The occurrence of a spike, entered as a time-dependent covariate in proportional hazards regression, significantly predicted development of agranulocytosis (risk ratio, 3.02; 95% confidence interval, 1.38 to 6.57). Spikes were sensitive though nonspecific predictors, occurring frequently in patients who did not develop agranulocytosis. These results, while exploratory, indicate the potential usefulness of these spikes as guidelines to govern the use of clozapine.

The use of clozapine has been limited by the risk of agranulocytosis. The cumulative incidence of agranulocytosis is 0.80% after 1 year and 0.91% after 1 1/2 years. Risk is greatest during the first 3 months of treatment and greater among women and the elderly. White-cell counts tend to spike upward before the occurrence of agranulocytosis. The occurrence of a white-cell count spike of > or = 15% above the previous count predicts development of agranulocytosis within 75 days (risk ratio = 3.02; 95% confidence interval, 1.38-6.57). Our results indicate that hematologic monitoring is necessary and effective. They also suggest that the frequency of monitoring could be reduced after the initial period of maximal risk.

Background: Clozapine is an atypical antipsychotic agent that is more effective than standard neuroleptic drugs in the treatment of patients with refractory schizophrenia. Unlike classic neuroleptic agents, clozapine is not associated with the development of acute extrapyramidal symptoms or tardive dyskinesia. The main factor limiting its use is the risk of potentially fatal agranulocytosis, estimated to occur in 1 to 2 percent of treated patients. After clozapine was approved by the Food and Drug Administration, it became available for marketing in the United States in February 1990 only as part of a special surveillance system (the Clozaril Patient Management System, or CPMS), in which a weekly white-cell count was required for the patient to receive a supply of the drug., Methods: We evaluated the CPMS data for February 1990 through April 1991 by survival analysis to determine the incidence of agranulocytosis and the effects of potential risk factors such as age and sex. Data were available for 11,555 patients who received clozapine during the period after marketing began., Results: Agranulocytosis developed in 73 patients, resulting in death from infectious complications in 2 patients. Episodes of agranulocytosis occurred in 61 patients within three months after they began treatment. The cumulative incidence of this side effect was 0.80 percent (95 percent confidence interval, 0.61 to 0.99) at 1 year and 0.91 percent (95 percent confidence interval, 0.62 to 1.20) at 1 1/2 years. The risk of agranulocytosis increased with age and was higher among women., Conclusions: The occurrence of agranulocytosis is a substantial hazard of the administration of clozapine, but this hazard can be reduced by monitoring the white-cell count. The increasing risk of agranulocytosis with age and the reduced incidence after the first six months of treatment provide additional guidelines for the prescription and monitoring of clozapine treatment in the future.