A 35-year-old woman with a 2-year history of progressive fatigue presents with a hemoglobin of 7.2 g/dL. Peripheral blood smear shows microcytic, hypochromic RBCs with target cells and occasional nucleated RBCs. Which single feature best distinguishes iron deficiency anemia from thalassemia trait on the same smear?

Explanation

## Distinguishing Iron Deficiency Anemia from Thalassemia Trait ### The Mentzer Index **High-Yield:** The Mentzer index (MCV ÷ RBC count) is the single best discriminator between iron deficiency anemia and thalassemia trait when both present with microcytic anemia. **Key Point:** - **Mentzer index < 13** → Thalassemia trait (RBC count is disproportionately high relative to MCV) - **Mentzer index > 13** → Iron deficiency anemia (RBC count is relatively lower for the degree of microcytosis) This occurs because thalassemia trait produces many small RBCs (high RBC count despite low MCV), whereas iron deficiency produces fewer, smaller RBCs (lower RBC count with low MCV). ### Overlapping Smear Features | Feature | Iron Deficiency | Thalassemia Trait | Discriminatory? | |---------|-----------------|-------------------|------------------| | Target cells | Present | Present | No | | Microcytic hypochromic RBCs | Yes | Yes | No | | Nucleated RBCs | Rare (unless severe) | Rare | No | | RBC count | Low-normal to low | **Elevated** | Yes | | MCV | Low | Low | No | **Clinical Pearl:** Both conditions show target cells and microcytic hypochromic RBCs on smear. The **RBC count** is the key: thalassemia trait maintains a relatively high RBC count despite low MCV, whereas iron deficiency shows a lower RBC count. The Mentzer index mathematically captures this distinction. ### Why Smear Morphology Alone Is Insufficient Morphologic features (target cells, hypochromia, microcytosis) overlap significantly. Discrimination requires a **numerical parameter** — the Mentzer index — which integrates MCV and RBC count. [cite:Robbins 10e Ch 13]