About the Mentzer Index
The Mentzer Index is a simple ratio, described by W.C. Mentzer Jr. in a 1973 Lancet letter, that helps distinguish two common causes of microcytic (small red blood cell) anemia: iron deficiency anemia and beta-thalassemia trait. Both conditions can produce a low mean corpuscular volume (MCV), but they tend to behave differently with respect to the red blood cell count, and the index turns that difference into a single screening number from a routine complete blood count (CBC).
How the index is calculated
The formula divides the mean corpuscular volume (MCV, reported in femtoliters) by the red blood cell count (RBC, reported in millions of cells per microliter):
Mentzer Index = MCV ÷ RBC
For example, an MCV of 68 fL with an RBC count of 5.4 million cells/uL gives an index of about 12.6.
Interpreting the result
In Mentzer's original description, an index below 13 is more consistent with beta-thalassemia trait, where the RBC count is often normal or elevated despite small cells. An index of 13 or higher is more consistent with iron deficiency anemia, where the RBC count tends to fall alongside the MCV. Results that land close to the cutoff of 13 are less discriminating than values well above or below it, so treat borderline numbers cautiously.
Limitations
The Mentzer Index uses only two CBC values and does not account for mixed anemias, other hemoglobinopathies, recent transfusion, or early iron deficiency where the red blood cell indices have not yet fully shifted. It is a bedside screening aid, not a substitute for confirmatory testing.
When to consult a professional
This calculator performs the standard Mentzer arithmetic for education and quick screening. It does not diagnose anemia or direct treatment. Confirming the underlying cause of a microcytic anemia - and deciding on any iron supplementation or further hematology workup - should be done with a clinician using additional tests such as ferritin, iron studies, or hemoglobin electrophoresis.