## Clozapine-Induced Agranulocytosis **Key Point:** Agranulocytosis (absolute neutrophil count <500/μL) is the most serious and life-threatening side effect of clozapine, occurring in 0.5–2% of patients. ### Mechanism Clozapine causes dose-dependent, idiosyncratic bone marrow suppression: - **Direct myelotoxicity** — clozapine metabolites damage hematopoietic stem cells - **Immune-mediated** — possible antibody-mediated destruction of neutrophils - **Peak incidence** — first 4–12 weeks of treatment (80% of cases occur within 3 months) ### Monitoring Requirements (Mandatory) | Time Period | ANC Monitoring Frequency | Action if ANC <1500/μL | | --- | --- | --- | | First 6 months | Weekly | Discontinue clozapine; recheck ANC | | Months 6–12 | Biweekly | Discontinue if <1500/μL | | After 12 months | Monthly | Discontinue if <1000/μL | | After 2 years (stable) | Every 4 weeks | May extend to monthly | **High-Yield:** Clozapine MUST NOT be initiated or continued without: 1. Baseline ANC ≥1500/μL (or ≥1000/μL if history of benign ethnic neutropenia) 2. Regular ANC monitoring per protocol 3. Patient and prescriber enrollment in REMS (Risk Evaluation and Mitigation Strategy) program ### Clinical Presentation of Agranulocytosis - Fever, sore throat, oral ulcers - Severe infection (sepsis, pneumonia) - Rapid onset (hours to days) - Mortality if untreated: 5–15% **Warning:** Do NOT re-challenge clozapine after agranulocytosis; risk of recurrence is very high (>80%). ### Why Clozapine Despite the Risk? Clozapine remains the **gold standard for treatment-resistant schizophrenia** (defined as failure of ≥2 antipsychotics at therapeutic doses). Its superior efficacy in reducing negative symptoms and suicide risk justifies the risk with careful monitoring. [cite:Stahl's Essential Psychopharmacology 6e Ch 5; American Psychiatric Association Practice Guideline for Schizophrenia]
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