Prevalence and frequency of haematological testing in patients initiating clozapine in the USA

BACKGROUND: Until June 2025, individuals initiating clozapine in the USA were required to undergo routine haematological testing in accordance with the Clozapine Risk Evaluation and Mitigation Strategy (REMS) programme. Although the Clozapine REMS programme was eliminated in June 2025, clinical and prescribing recommendations on testing remained intact. Little is known, however, about adherence to testing requirements in practice. OBJECTIVE: The aim of this study was to assess the prevalence and frequency of haematological testing in patients on clozapine. METHODS: A cohort study using three US health insurance claims databases (1 January 2000-31 March 2022) assessed (1) proportion of individuals with at least one haematological test (white blood cell count with differential) prior to clozapine initiation and (2) frequency of testing (number of tests per 30 days of follow-up) during months 0-6, 7-12 and 13-24 of clozapine therapy. Tests were identified based on billing codes; patients hospitalised during the assessment period were assumed to have received testing. FINDINGS: A total of 62 003 clozapine initiations (mean (SD) age, 36 (15) years; 57% male) were identified, of which 71% were preceded by testing within 60 prior days. The mean (SD) testing rate while on therapy was 2.2 (1.8) tests per patient for each 30-day period during the first 6 months of treatment, 2.0 (1.8) tests per 30 days during months 7-12 and 1.5 (1.4) tests per 30 days during months 13-24. CONCLUSIONS: In this study of individuals initiating clozapine in the USA, haematological testing was completed at a lower-than-expected rate prior to clozapine initiation and during the first 6 months of treatment but was in accordance with testing requirements thereafter. CLINICAL IMPLICATIONS: Although clozapine drug labelling recommends routine haematological testing, especially during the first 6 months of treatment, such testing may not consistently occur. Reasons for less-than-recommended testing frequency should be further investigated.