A 32-year-old woman with Graves' disease has been on propranolol and propylthiouracil (PTU) for 3 weeks. She now presents with fever, sore throat, and a white blood cell count of 1,200/μL. Which investigation is most appropriate to confirm the suspected adverse effect?

## Clinical Context The patient presents with fever, sore throat, and leukopenia (WBC 1,200/μL) after 3 weeks of PTU therapy. This constellation is highly suggestive of **agranulocytosis**, a rare but serious idiosyncratic adverse effect of PTU (and other antithyroid drugs). ## Investigation of Choice **Key Point:** Absolute neutrophil count (ANC) is the most appropriate investigation to confirm agranulocytosis. An ANC < 500/μL in the setting of fever and infection confirms the diagnosis. **High-Yield:** Agranulocytosis due to PTU typically occurs within the first 3 months of therapy and is dose-independent (idiosyncratic). The reticulocyte count helps assess bone marrow response and recovery potential. ## Why ANC + Reticulocyte Count? 1. **ANC quantifies the severity** of neutropenia and confirms agranulocytosis (ANC < 500/μL is diagnostic). 2. **Reticulocyte count** assesses whether the bone marrow is capable of regenerating neutrophils (high reticulocyte count suggests recovery; low count suggests marrow suppression). 3. Both are **non-invasive, rapid, and cost-effective** — essential in an acute clinical emergency. 4. Together they guide prognosis and management (G-CSF, antibiotics, discontinuation of PTU). ## Why NOT Bone Marrow Biopsy? Bone marrow aspiration is **not the first-line investigation** in suspected agranulocytosis. It is invasive, time-consuming, and not necessary for diagnosis — the clinical presentation + ANC < 500/μL is sufficient. Marrow biopsy is reserved for cases with unclear etiology or failure to recover after 5–7 days of supportive care. ## Why NOT Peripheral Blood Smear? A peripheral smear may show toxic granulations and left shift if there is an acute infection, but it does **not quantify the absolute number of neutrophils** and is less reliable than ANC for confirming agranulocytosis. ## Why NOT Flow Cytometry? Flow cytometry (CD34+ cells) is used to diagnose acute leukemias or assess hematopoietic stem cell populations — not relevant to drug-induced agranulocytosis. ## Clinical Pearl **Mnemonic: PTU Agranulocytosis — "FEVER + LOW WBC + PTU = STOP PTU immediately"** - PTU should be discontinued immediately upon suspicion. - Supportive care: broad-spectrum antibiotics, G-CSF if ANC < 200/μL and sepsis risk is high. - Switch to propranolol + iodine solution (Lugol's) or beta-blocker + radioactive iodine therapy.