A 42-year-old woman with treatment-resistant schizophrenia started on clozapine 3 weeks ago now presents with fever, sore throat, and oral ulcers. Her white blood cell count is 2100/μL with absolute neutrophil count (ANC) of 800/μL. Which investigation is most appropriate to assess the risk of progression to agranulocytosis?

## Clozapine-Induced Agranulocytosis: Monitoring Strategy Clozapine carries a 0.5–1% risk of agranulocytosis (ANC <500/μL), a potentially fatal complication. Early detection through serial ANC monitoring is the cornerstone of safe clozapine use. ### Investigation of Choice: Serial ANC Monitoring **Key Point:** Serial ANC counts are the ONLY investigation that can detect the rapid decline in neutrophils characteristic of clozapine-induced agranulocytosis. This allows timely drug discontinuation before life-threatening infection occurs. **High-Yield:** Clozapine-induced agranulocytosis typically develops rapidly (over days to weeks) and is reversible if caught early. Serial monitoring is mandatory, not optional. ### Monitoring Schedule for Clozapine | Phase | ANC Monitoring Frequency | Action Threshold | |-------|--------------------------|------------------| | **Week 1–4** | Baseline + every 3 days | Stop if ANC <1500/μL | | **Week 5–12** | Weekly | Stop if ANC <1500/μL | | **After 12 weeks** | Every 4 weeks (if stable) | Stop if ANC <1500/μL | | **Agranulocytosis** | ANC <500/μL | Immediate discontinuation, hospitalization | **Clinical Pearl:** The patient's current ANC of 800/μL is dangerously low and approaching agranulocytosis (<500/μL). Clozapine must be stopped immediately, and serial ANC monitoring should continue every 1–2 days until recovery. **Mnemonic: CLOZ-ANC** — **C**lozapine requires **L**abor-intensive **O**ngoin**Z** **A**bsolute **N**eutrophil **C**ount monitoring. ### Why Serial ANC Monitoring Is the Gold Standard 1. **Detects rapid decline:** Agranulocytosis can develop in days; serial counts catch this 2. **Guides discontinuation:** Allows timely drug withdrawal before infection 3. **Reversible if caught early:** Stopping clozapine usually restores ANC within 1–2 weeks 4. **Prevents mortality:** Early detection prevents sepsis and death **Warning:** A single low ANC is not diagnostic of agranulocytosis—the trend (rapid decline) is what matters. Serial monitoring reveals the trajectory. [cite:KD Tripathi 8e Ch 12; Harrison 21e Ch 385]