An asymptomatic 24-year-old woman is found on routine testing to have Hb 10.8 g/dL, MCV 66 fL, RBC count 6.4 million per microliter, and normal serum ferritin and iron studies. Using the Mentzer index, what is the most likely explanation?

• A Sideroblastic anemia
• B Iron deficiency anemia
• C Anemia of chronic disease
• D Beta thalassemia trait ✓

Explanation

The Mentzer index is MCV divided by RBC count in millions. Here 66 divided by 6.4 gives about 10.3, below 13, pointing to thalassemia trait, where the red cell count is paradoxically preserved or high relative to the small MCV. In iron deficiency the count falls along with the MCV, giving a ratio above 13. Normal ferritin and iron studies independently exclude iron deficiency and anemia of chronic disease, and sideroblastic anemia typically shows ringed sideroblasts with high rather than low serum iron.

Reference: Williams Hematology, 10th ed.

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