Mentzer Index Calculator (MCV ÷ RBC: thalassaemia trait vs iron deficiency)

Q: Is thalassaemia trait harmful?

It usually causes no symptoms or only mild anaemia, but it matters for family planning.

Q: Are there other indices?

Yes, several such as the Green and King index, but none replaces specific tests.

Understanding the Mentzer index calculator — concept and principle

The Mentzer index divides the mean corpuscular volume (MCV) by the red blood cell count. It gives a quick hint about whether small red cells (microcytosis) are more likely due to thalassaemia trait or iron deficiency, the two most common causes, before specific tests are done.

MCV ÷ red cell count: with the default inputs the result is 12.1 < 13 (thalassaemia trait more likely).

How it is calculated

Mentzer index = MCV (fL) ÷ RBC (millions per µL)

Below 13: thalassaemia trait more likely.
Above 13: iron deficiency more likely.
The index is meant for people with small red cells (MCV under 80 fL).

The principle

In thalassaemia trait the bone marrow makes a normal or even increased number of red cells, but each is small because of reduced globin production. The RBC count stays normal or high while the MCV is low, so the ratio is small. In iron deficiency the marrow lacks the iron to make enough haemoglobin, so it makes fewer cells as well as smaller ones; the RBC count falls along with the MCV, and the ratio stays higher.

Worked example

Default inputs: MCV 68 fL, RBC 5.6 million/µL.

Mentzer index = 68 ÷ 5.6 = 12.14 → 12.1.
12.1 is below 13 → Thalassaemia trait more likely.
MCV 68 < 80 → small red cells Yes.

Each point is the calculator re-run with only “Red blood cell count (×10⁶/µL = ×10¹²/L)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Red blood cell count (×10⁶/µL = ×10¹²/L)	Mentzer index
1.4	48.6
2.8	24.3
4.2	16.2
5.6	12.1
7	9.7
8.4	8.1
11.2	6.1

How to read the result

Where does your result fall?

The index is a screening hint, not a diagnosis. Iron deficiency is confirmed with ferritin. Beta-thalassaemia trait is confirmed with haemoglobin electrophoresis or HPLC, usually showing raised HbA2, and alpha-thalassaemia trait often needs DNA testing. Both conditions can exist together, and iron deficiency can mask a raised HbA2.

Common mistakes

Using the index when the MCV is normal.
Relying on it in pregnancy, young children or mixed anaemia, where it is less reliable.
Starting iron without checking ferritin.

Keep in mind: This is not a diagnosis. Anaemia and small red cells should be investigated by a doctor.

Frequently asked questions

What Mentzer index suggests thalassaemia?

A value below 13.

Is thalassaemia trait harmful?