What does the Mentzer index show?

It helps tell apart two causes of microcytic anemia: beta-thalassemia (index 13). It’s a screening guide from a complete blood count, not a final diagnosis.

How do I calculate the Mentzer index?

Index = MCV ÷ RBC, where MCV is the mean corpuscular volume (fL) and RBC the red blood cell count (×10¹²/L). Both come from a complete blood count; the calculator does the math.

Is an index below 13 thalassemia?

An index below 13 raises the likelihood of beta-thalassemia but doesn’t confirm it. Hemoglobin electrophoresis and iron stores are needed. A doctor makes the final call.

Is an index above 13 iron deficiency?

A value above 13 leans toward iron deficiency anemia, confirmed by low ferritin and transferrin saturation. The index just points where to look.

How accurate is the Mentzer index?

It’s handy but not perfect: it errs with mixed deficiencies, a normal MCV or after transfusion. So it’s assessed in context, not in isolation.

Mentzer index: calculator and interpretation

The Mentzer index helps tell apart two common causes of microcytic anemia (small red cells): beta-thalassemia and iron deficiency. Enter the mean corpuscular volume (MCV) and red blood cell count (RBC) from a complete blood count — the calculator computes the index: below 13 points to thalassemia, above 13 to iron deficiency.

Calculate the Mentzer index

Enter MCV and red blood cell count — the result appears instantly.

How to read the Mentzer index

The index distinguishes causes of microcytosis but doesn’t diagnose: confirm with ferritin, transferrin saturation and, if thalassemia is suspected, hemoglobin electrophoresis.

Mentzer index	Likely cause
below 13	Beta-thalassemia likely
around 13	Indeterminate — needs more tests
above 13	Iron deficiency likely

What the Mentzer index is and how it’s computed

The Mentzer index = MCV (mean corpuscular volume, fL) ÷ RBC (red blood cell count, ×10¹²/L). Both values are in a routine complete blood count.

The idea is simple: in thalassemia the marrow makes many small red cells (high RBC, low MCV → small index), while in iron deficiency there are few red cells (low RBC → large index).

Telling thalassemia from iron deficiency

An index below 13 leans toward beta-thalassemia (trait), above 13 toward iron deficiency anemia. A value around 13 is indeterminate and needs other tests.

It’s a screening guide, not a diagnosis: accuracy isn’t 100%. Iron deficiency is confirmed with ferritin and transferrin saturation, thalassemia with hemoglobin electrophoresis and, if needed, genetic testing.

When the index doesn’t work

The Mentzer index is designed for microcytic anemia (low MCV). With a normal or high MCV, mixed deficiencies (iron + B12/folate), recent transfusion or acute blood loss, it can mislead.

So it’s always assessed together with the full blood count and iron stores, and the decision is made by a doctor.

Frequently asked questions

It helps tell apart two causes of microcytic anemia: beta-thalassemia (index < 13) and iron deficiency (index > 13). It’s a screening guide from a complete blood count, not a final diagnosis.
Index = MCV ÷ RBC, where MCV is the mean corpuscular volume (fL) and RBC the red blood cell count (×10¹²/L). Both come from a complete blood count; the calculator does the math.
An index below 13 raises the likelihood of beta-thalassemia but doesn’t confirm it. Hemoglobin electrophoresis and iron stores are needed. A doctor makes the final call.
A value above 13 leans toward iron deficiency anemia, confirmed by low ferritin and transferrin saturation. The index just points where to look.
It’s handy but not perfect: it errs with mixed deficiencies, a normal MCV or after transfusion. So it’s assessed in context, not in isolation.

Decode your complete blood count

Upload your CBC — AI reads red cells, MCV, hemoglobin and iron stores together and explains what microcytosis means in your case.

Decode tests

This calculator is for reference and information only and is not a diagnosis. The cause of anemia is determined by a doctor from the complete blood count, ferritin and, if needed, hemoglobin electrophoresis.