In the workup of microcytic anemia, which of the following is the FIRST and most cost-effective screening test to differentiate iron deficiency anemia from thalassemia trait?

Explanation

## Microcytic Anemia Workup Algorithm **Key Point:** The RDW/MCV ratio (Mentzer index) is the simplest, most cost-effective initial discriminator between iron deficiency anemia (IDA) and thalassemia trait in microcytic anemia. ### Mentzer Index Formula $$\text{Mentzer Index} = \frac{MCV}{RDW}$$ - **Mentzer Index > 13** → Suggests thalassemia trait (uniform RBC size) - **Mentzer Index < 13** → Suggests iron deficiency anemia (heterogeneous RBC sizes) ### Why This Test First? | Feature | IDA | Thalassemia Trait | |---------|-----|-------------------| | **RDW** | Elevated (>15%) | Normal (11–15%) | | **MCV** | Low (<75 fL) | Low (<75 fL) | | **RDW/MCV ratio** | Low (<13) | High (>13) | | **Cost** | Minimal (CBC already done) | Minimal | | **Sensitivity** | High | High | **High-Yield:** Once Mentzer index suggests the diagnosis, confirm with: - IDA: serum ferritin, serum iron, TIBC - Thalassemia trait: hemoglobin electrophoresis (HbA₂ >3.5%, HbF elevated) ### Why Not the Other Options? **Serum ferritin** is specific for IDA but does NOT differentiate from thalassemia; it is confirmatory, not screening. **Hemoglobin electrophoresis** is diagnostic for thalassemia but requires prior suspicion; using it first is inefficient and costly. **Bone marrow biopsy** is invasive and unnecessary in routine microcytic anemia workup; reserved for refractory cases or diagnostic uncertainty after labs. **Clinical Pearl:** In Indian populations, thalassemia trait is common; the RDW/MCV ratio is especially valuable to avoid unnecessary iron supplementation in thalassemia carriers (which can cause secondary hemochromatosis).