A 28-year-old woman with heavy menstrual bleeding presents with fatigue. CBC shows Hb 9.2 g/dL, MCV 68 fL, and RBC count 5.2 × 10^6/μL. Serum ferritin is 8 ng/mL. Which investigation would be most useful to differentiate iron deficiency anemia from thalassemia trait in this patient?

Explanation

## Differentiating Iron Deficiency from Thalassemia Trait ### The Mentzer Index: A Simple Discriminator **Key Point:** The Mentzer index (MCV/RBC count) is a simple, cost-effective calculation that distinguishes iron deficiency anemia from thalassemia trait: - **Mentzer index <13** = Thalassemia trait (low MCV with relatively preserved RBC count) - **Mentzer index >13** = Iron deficiency anemia (low MCV with low RBC count) **High-Yield:** In this case: - Mentzer index = 68 / 5.2 = **13.1** → Suggests iron deficiency anemia - The high RBC count (5.2 × 10^6/μL) despite low MCV is typical of thalassemia; iron deficiency usually shows both low MCV AND low RBC count ### Why Mentzer Index Works | Feature | Iron Deficiency | Thalassemia Trait | |---|---|---| | MCV | ↓ (often <70) | ↓ (often 60–70) | | RBC count | ↓ | Normal or ↑ | | Mentzer index (MCV/RBC) | >13 | <13 | | Serum ferritin | ↓ | Normal | | TIBC | ↑ | Normal | **Clinical Pearl:** The Mentzer index is useful as a **screening tool** but is not diagnostic. In this patient, the low ferritin (8 ng/mL) already strongly suggests iron deficiency. The Mentzer index provides additional support and helps exclude thalassemia trait, which would present with normal ferritin. **Mnemonic: MENT-13** — Mentzer index <13 = thalassemia, >13 = iron deficiency