What is the Mentzer Index?
The Mentzer Index is a simple screening calculation that helps distinguish iron deficiency anemia from thalassemia trait when a CBC shows microcytic anemia (MCV below 80 fL). It was described by Mentzer in 1973 and remains useful because both conditions look similar on a basic CBC, but need very different next steps — iron studies for one, hemoglobin electrophoresis or genetic testing for the other.
How to calculate it
Mentzer Index = MCV (fL) ÷ RBC count (×10⁶/µL)
The logic: in iron deficiency, the bone marrow makes fewer, smaller red cells — both MCV and RBC count drop together. In thalassemia trait, the marrow compensates for the genetic defect by making more, smaller red cells — MCV drops but RBC count stays normal or rises. Dividing MCV by RBC count captures this difference in a single number.
Interpretation
| Mentzer Index | Suggests | Next step |
|---|---|---|
| > 13 | Iron deficiency anemia | Iron studies (ferritin, serum iron, TIBC) |
| < 13 | Thalassemia trait | Hemoglobin electrophoresis or genetic testing |
This only applies once microcytic anemia has already been identified (MCV <80 fL) — it isn't meaningful in normocytic or macrocytic anemia. For the broader anemia workup, see the CBC Interpretation & Anemia Approach guide.
References
Mentzer WC Jr. Differentiation of iron deficiency from thalassaemia trait. Lancet. 1973.