Background: In February 2025, the FDA ended the mandatory Risk Evaluation and Mitigation

Strategy requirements for clozapine. Korea continues mandatory hematologic testing under

the Clozapine Patient Monitoring System (CPMS), yet no nationwide updates have been

reported since 2006.

Objectives: This study aimed to provide contemporary real-world evidence on the incidence,

timing, and clinical outcomes of clozapine-associated neutropenia and agranulocytosis in

Korea, and to evaluate adherence to monitoring regulations, to inform potential revision of the

national CPMS.

Design: A retrospective, multicenter cohort study was conducted using anonymized electronic

medical records from two Korean tertiary hospitals between 2000 and 2023.

Methods: Patients with schizophrenia spectrum disorders who had newly initiated clozapine

were included. Absolute neutrophil count (ANC) data were used to identify neutropenia

(ANC < 1500/μL), moderate neutropenia (<1000/μL), and agranulocytosis (<500/μL). Chart

review determined causality for agranulocytosis. Monitoring adherence was assessed based

on the proportion of ANC tests conducted within recommended intervals.

Results: Among 2417 newly initiated patients (from 3364 total users), neutropenia occurred

in 9.7%. Of these, 47.4% occurred within 18 weeks, and none who continued treatment

progressed to agranulocytosis. Patients with neutropenia had lower baseline ANC than those

without. Moderate neutropenia occurred in 1.7% and showed similar clinical patterns, most

events being transient. Ten cases of agranulocytosis were identified, of which four (0.17%)

were clozapine-related. All appeared abruptly within 5 weeks of initiation without preceding

neutropenia or low baseline ANC, and recovered after discontinuation. Monitoring adherence

was suboptimal: during the first 18 weeks, adherence to ⩽10-day interval was 73.7%, while

8.7% patients had no ANC tests; adherence after 1 year fell below 50%.

Conclusion: Neutropenia is relatively common but usually low-risk, whereas clozapineinduced

agranulocytosis is rare, abrupt, and confined to the early period. These findings

suggest reconsideration of current hematological monitoring regulations regarding clozapine.

Keywords: agranulocytosis, clozapine, neutropenia, schizophrenia