Prevalence and Frequency of Haematological Testing in Patients Initiating Clozapine in the USA

Among more than 54,000 patients initiating clozapine from 2000 to 2022, roughly 30% had no evidence of blood testing in the 60 days before therapy began, and testing during the first 6 months averaged 2.2 tests per 30 days, well below the 4 required under the FDA’s clozapine Risk Evaluation and Mitigation Strategy (REMS). Testing rates were more in line with REMS requirements during months 7 through 24. The FDA eliminated the clozapine REMS in June 2025, but monitoring recommendations remain in the drug’s labeling, so the reasons for less-than-expected early testing warrant further investigation.