The Mentzer index is a simple bedside ratio — MCV divided by RBC count — that helps separate two common causes of microcytic anemia: thalassemia trait and iron deficiency anemia. This calculator computes the ratio and classifies it against the standard 13 cutoff.

How the Mentzer index works

The Mentzer index is simply MCV divided by RBC count: Mentzer index = MCV ÷ RBC count. Mentzer WC Jr. first described it in a 1973 Lancet letter as a fast way to separate two common causes of microcytic anemia using values already reported on a standard complete blood count (CBC).

The two conditions push the ratio in opposite directions. In iron deficiency, the bone marrow cannot produce as many red blood cells, so both the RBC count and the MCV fall together, pushing the ratio above 13. In thalassemia trait, red blood cell production is largely preserved even though the cells themselves are small, so the RBC count often stays normal-to-high relative to the low MCV, keeping the ratio below 13.

Inputs and what they mean

MCV (fL) is the average size of a red blood cell, reported on a CBC.

RBC count (x10^6/uL) is the number of red blood cells per microliter of blood, also reported on the same CBC. Both values should come from the same blood draw — mixing values from different dates can produce a misleading ratio.

Limits and edge cases

The Mentzer index is a screening heuristic, not a diagnosis. Reported sensitivity and specificity vary by population, roughly 60-90%. A patient with combined iron deficiency and thalassemia trait can produce a misleading result, since the two conditions push the ratio in opposite directions. The original index was validated against beta-thalassemia trait, so performance in alpha-thalassemia trait and other hemoglobinopathies is less consistent. A result on either side of 13, and especially one close to the cutoff, should be confirmed with iron studies (ferritin, serum iron, total iron-binding capacity) or hemoglobin electrophoresis rather than treated as conclusive on its own.