A 32-year-old woman with Graves' disease is started on propylthiouracil (PTU) therapy. After 6 weeks of treatment, she develops fever, sore throat, and a petechial rash. Her WBC count is 2,100/μL with severe neutropenia. Which investigation is most appropriate to confirm the suspected drug-induced complication?

## Clinical Scenario Analysis The patient presents with **fever, sore throat, petechial rash, and severe neutropenia (WBC 2,100/μL)** following PTU initiation—a classic presentation of **drug-induced agranulocytosis**, one of the most serious adverse effects of PTU. ## Investigation of Choice **Key Point:** Bone marrow biopsy and aspiration is the gold standard confirmatory test for agranulocytosis. It directly visualizes bone marrow cellularity and rules out other causes of neutropenia (aplastic anemia, leukemia, myelodysplasia). ### Why Bone Marrow Biopsy? | Feature | Significance | |---------|-------------| | **Direct visualization** | Confirms absence of myeloid precursors (maturation arrest) | | **Differential diagnosis** | Excludes aplasia, infiltration, or malignancy | | **Prognosis assessment** | Evaluates marrow reserve and recovery potential | | **Gold standard** | Only definitive test for agranulocytosis | ## PTU-Induced Agranulocytosis: Key Facts **High-Yield:** PTU carries a **0.1–0.5% risk** of agranulocytosis, typically occurring within the first 3 months of therapy. This is an **idiosyncratic reaction**, not dose-dependent. **Mnemonic: AGRANULOCYTOSIS** — **A**cute onset, **G**ranulocytes absent, **R**apid decline, **A**ntithyroid drugs (PTU > methimazole), **N**eutropenia severe, **U**rgent marrow assessment, **L**ife-threatening, **O**ften irreversible if untreated, **C**onfirm with biopsy, **Y**ield recovery if caught early, **T**oxic idiosyncratic, **O**bserve CBC weekly, **S**top drug immediately, **I**ntensive support, **S**epsis prevention. **Clinical Pearl:** Methimazole causes agranulocytosis in ~0.01% of patients (lower risk than PTU), but PTU is preferred in pregnancy due to lower teratogenicity. However, PTU's agranulocytosis risk has led many centers to reserve it for first-trimester use only. ## Why Other Investigations Are Insufficient **Serum TSH and free T4:** These assess thyroid function and PTU efficacy, not the hematologic complication. **ANCA panel:** While PTU-induced ANCA-associated vasculitis is a recognized complication, ANCA serology is a screening test, not confirmatory for agranulocytosis. Bone marrow biopsy is still needed to assess marrow function. **Peripheral blood smear and reticulocyte count:** These provide descriptive information (morphology, reticulocyte response) but do NOT confirm the diagnosis or assess marrow cellularity. A smear may show absence of bands/segmented neutrophils, but marrow biopsy is required for definitive diagnosis. ## Management Implications 1. **Immediate action:** Stop PTU immediately 2. **Supportive care:** Broad-spectrum antibiotics, G-CSF (granulocyte colony-stimulating factor) if marrow shows maturation arrest 3. **Monitoring:** Daily CBC until recovery 4. **Prognosis:** ~90% recover with prompt intervention; mortality if untreated is high **Warning:** Do NOT re-challenge with PTU or methimazole after agranulocytosis—cross-reactivity is possible. Consider radioiodine or surgery for definitive therapy.