A female patient presented with fatigue and a history of piles. Routine complete blood count analysis showed hemoglobin of 9 g/dL, MCV 60fL, and RBC count of 5.2 million. A peripheral smear is given below. Which of the following is the next best investigation for this patient? [image]

Explanation

## Correct Answer: B. Serum ferritin levels This patient presents with **microcytic anemia** (Hb 9 g/dL, MCV 60 fL) with a **normal-to-high RBC count** (5.2 million), which is the classic pattern of **iron deficiency anemia (IDA)**. The elevated RBC count despite low hemoglobin reflects the bone marrow's compensatory response to iron deficiency—producing more RBCs of smaller size to maintain oxygen-carrying capacity. The clinical history of piles (hemorrhoids) provides the bleeding source explaining chronic iron loss. The peripheral smear would show **hypochromic microcytic RBCs with anisocytosis and poikilocytosis**. In IDA, the next diagnostic step after confirming microcytic anemia is to measure **serum ferritin**, which is the most sensitive and specific marker of total body iron stores. Ferritin <15 ng/mL confirms iron deficiency. This is the standard approach per Indian guidelines and Harrison's protocol: once microcytic anemia is identified, ferritin estimation guides confirmation before empirical iron supplementation. In the Indian context, IDA from chronic GI bleeding (piles, hookworm, PUD) is the most common cause of anemia, making ferritin the logical next step to confirm the diagnosis and quantify iron depletion severity. ## Why the other options are wrong **A. Serum folate levels** — Folate deficiency causes **macrocytic anemia** (elevated MCV >100 fL), not microcytic anemia. The MCV of 60 fL rules out folate deficiency. This is an NBE trap that tests whether students confuse the MCV pattern with nutritional deficiencies—folate and B12 deficiency are macrocytic, not microcytic. **C. HbA2 levels** — HbA2 estimation is used to diagnose **β-thalassemia trait**, which also presents with microcytic anemia and elevated RBC count. However, thalassemia trait typically has **normal-to-elevated ferritin** (no iron loss) and a **normal serum iron**. The clinical history of piles with chronic bleeding makes iron deficiency far more likely. HbA2 is checked only if ferritin is normal and iron studies are inconclusive. **D. Serum homocysteine levels** — Elevated homocysteine indicates **B12 or folate deficiency**, both causing **macrocytic anemia**, not microcytic. The MCV of 60 fL excludes B12 deficiency. Homocysteine testing is irrelevant in microcytic anemia and represents a distractor testing knowledge of macrocytic anemia workup. ## High-Yield Facts - **Microcytic anemia with elevated RBC count** = iron deficiency anemia until proven otherwise; the high RBC count reflects compensatory erythropoiesis. - **Serum ferritin <15 ng/mL** confirms iron deficiency; ferritin is the most sensitive marker of total body iron stores. - **Chronic GI bleeding** (piles, hookworm, PUD) is the leading cause of IDA in India; ferritin guides severity and transfusion need. - **Macrocytic anemia** (MCV >100 fL) = folate/B12 deficiency; **microcytic anemia** (MCV <80 fL) = iron deficiency or thalassemia. - **β-thalassemia trait** also presents with microcytic anemia + elevated RBC, but ferritin is normal; HbA2 >3.5% confirms thalassemia. ## Mnemonics **MCV-based anemia classification** **MICRO** (MCV <80): Iron, Thalassemia, Sideroblastic. **MACRO** (MCV >100): B12, Folate, Alcohol. **NORMO** (MCV 80–100): Hemolysis, Chronic disease, Bleeding. **IDA workup sequence** **CBC → Ferritin → Iron studies (serum iron, TIBC, transferrin saturation) → Peripheral smear → Source of bleeding**. Ferritin is the gatekeeper; if low, confirm with iron studies and find the bleeding source. ## NBE Trap NBE pairs microcytic anemia with folate/B12 deficiency (macrocytic) to test whether students confuse MCV patterns with nutritional deficiencies. The elevated RBC count is a red herring—students may think "high RBC = thalassemia," but the clinical history of piles and the ferritin test confirm iron deficiency. ## Clinical Pearl In Indian clinical practice, a patient with piles and microcytic anemia is iron deficient until proven otherwise. Ferritin <15 ng/mL confirms the diagnosis and quantifies iron depletion; this guides both iron supplementation dosing and the urgency of investigating the bleeding source (colonoscopy for hemorrhoids, upper endoscopy for PUD, stool microscopy for hookworm). _Reference: Harrison Ch. 98 (Anemia); Robbins Ch. 14 (Red Blood Cell Disorders); KD Tripathi Ch. 12 (Hematopoiesis)_