Mentzer Index Calculator (Thalassaemia vs Iron Deficiency)

How it works

Mentzer = MCV (fL) ÷ RBC (×10¹²/L); < 13 thalassaemia trait, > 13 iron deficiency. England-Fraser = MCV − RBC − 5 × Hb − 3.4 (< 0 thal). Green-King = MCV² × RDW ÷ (Hb × 100) (< 65 thal)

Both iron deficiency and thalassaemia trait produce small red cells, but they differ in how many cells the marrow makes. In thalassaemia trait the marrow churns out large numbers of small cells — the red cell count is high or normal — while in iron deficiency both cell size and cell number fall. Mentzer’s 1973 index simply divides MCV by the red cell count: below 13 suggests thalassaemia, above 13 iron deficiency. It is about 80–90% accurate and free, so it is a useful triage step before ordering ferritin and haemoglobin electrophoresis, especially in populations where both conditions are common. Several other discriminant indices exploit the same physiology plus red cell size variation (RDW), which is high in iron deficiency and normal in thalassaemia. None is definitive, and the two conditions frequently coexist.

Worked example

MCV 66 fL, RBC 5.6, Hb 11.8: Mentzer 11.8 — favours beta-thalassaemia trait; England-Fraser −2.0 and Green-King 49.8 agree, and the normal RDW of 13.5% fits.

Frequently asked questions