Late-Onset Neutropenia in Clozapine Users: Unrelated or Drug-Induced? A Case-Registry Analysis of Incidence, Characteristics, and Rechallenge Attempts

Background and Hypothesis Clozapine treatment carries a risk of blood dyscrasias (BD) and requires indefinite monitoring in many jurisdictions, a major factor in its under-utilization. Although previous studies suggest BD risk is highest early in treatment, BD events have also been reported after many years. This study compares early vs late (>6 months) suspected blood dyscrasias (SBD) and examines rechallenge outcomes as a marker for clozapine-related causation. Study Design A retrospective analysis of electronic health records from a large UK mental health service gathered demographic data, characteristics of SBD events, and outcomes of clozapine rechallenge, defined as reinitiation after SBD-related discontinuation. These variables were compared between early- and late-onset SBD groups using a 6-month treatment duration cutoff. Study Results Of 130 patients with SBD leading to clozapine cessation, 59 had early-onset SBD. The incidence rate before 6 months was 5.54% per year vs 0.53% after 6 months, reflecting an incidence rate ratio of 10.4. Early-onset patients were younger, received lower clozapine doses, and had fewer concurrent antipsychotics. Of 81 rechallenge attempts, 71 (87.7%) were successful, with a mean follow-up of 2.5 years. No significant differences in characteristics or rechallenge outcomes were found between the early- and late-onset groups. Conclusions Though less frequent, late-onset SBD shares similar characteristics with early-onset SBD and has a comparable risk of recurrence on clozapine rechallenge. Vast majority of clozapine rechallenges are successful, including early-onset BD, suggesting they are not clozapine-induced. However, clozapine-induced BD, defined by recurrence upon rechallenge, may rarely occur even after years of treatment.