Mentzer Index Calculator
The Mentzer index is a quick screening ratio that helps distinguish beta thalassemia trait from iron deficiency anemia, the two most common causes of microcytic anemia. It divides the mean corpuscular volume (MCV) by the red blood cell count (RBC). In thalassemia trait the red cell count tends to be preserved or high, giving a low index; in iron deficiency the red cell count is low, giving a high index. Enter MCV in femtoliters and RBC in millions per microliter. This is an educational screening aid, not a diagnosis or a replacement for confirmatory testing.
Mentzer index formula
Mentzer index = MCV / RBC count
MCV in femtoliters; RBC in millions per microliter
Below 13 suggests thalassemia trait
Above 13 suggests iron deficiency anemia
The ratio captures the observation that thalassemia trait preserves the red cell count despite small cells, lowering the index, while iron deficiency reduces the count, raising it.
Mentzer index context
- Below 13: thalassemia trait more likely.
- Around 13: indeterminate, further testing needed.
- Above 13: iron deficiency anemia more likely.
- It is a screening aid, not a confirmatory test.
- Confirm with iron studies and hemoglobin electrophoresis.
Mentzer index: frequently asked questions
How is the Mentzer index calculated?
The Mentzer index equals the mean corpuscular volume (MCV, in femtoliters) divided by the red blood cell count (RBC, in millions per microliter). It is a simple ratio used to help distinguish beta thalassemia trait from iron deficiency anemia in microcytic anemia.
How is the Mentzer index interpreted?
A value below 13 suggests thalassemia trait, where the RBC count is preserved or high. A value above 13 suggests iron deficiency anemia, where the RBC count is low. A value near 13 is indeterminate.
What units are used?
MCV is entered in femtoliters (fL) and the red blood cell count in millions per microliter (for example 5.2 for 5.2 million per uL). The index itself is unitless.
Is the Mentzer index definitive?
No. It is a screening aid, not a confirmatory test. Definitive diagnosis of thalassemia or iron deficiency requires further tests such as iron studies and hemoglobin electrophoresis, interpreted by a clinician.
Is this a diagnosis?
No. This calculator is for education. The Mentzer index supports, but does not replace, clinical evaluation and confirmatory laboratory testing.
Official sources
- National Library of Medicine (NCBI): Microcytic anemia discrimination indices.
- National Heart, Lung, and Blood Institute: Thalassemia and anemia information.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.