Mentzer Index Calculator
Mentzer Index Calculator
Screen a microcytic anaemia toward thalassaemia trait or iron deficiency using MCV and red cell count, before confirming with ferritin or haemoglobin electrophoresis.
Mentzer Index
Microcytosis screenMCV 65 fL, red cell count 5.8 ×10¹²/L
Formula
- MCV
- mean corpuscular volume, fL
- RBC
- red cell count, ×10¹²/L
- applies only to
- a microcytic anaemia — the index is not meaningful with a normal or high MCV
Worked example
MCV 65 fL, red cell count 5.8 ×10¹²/L
65 ÷ 5.8 = 11.2
Below 13, so beta-thalassaemia trait is more likely than iron deficiency
Why the ratio separates the two conditions
| Feature | Thalassaemia trait | Iron deficiency |
|---|---|---|
| Red cell count | Preserved or raised — the marrow makes extra cells to compensate | Low or low-normal — fewer cells are made overall |
| MCV | Low | Low |
| Mentzer Index | Typically below 13 | Typically above 13 |
| RDW | Usually normal — cells are uniformly small | Usually raised — cell size is variable |
| Confirmatory test | Haemoglobin electrophoresis or HPLC | Serum ferritin |
Many small cells versus few small cells
The Mentzer Index rests on a simple physiological difference between two conditions that both produce microcytosis. In beta-thalassaemia trait, a mild reduction in beta-globin chain synthesis leads to smaller cells, but total red cell production is preserved or even increased, so the red cell count stays normal or rises to compensate. In iron deficiency, the limiting factor is haemoglobin synthesis itself, so the marrow makes fewer cells overall as well as smaller ones. The result is that thalassaemia trait produces many small cells — a low MCV divided by a normal or high RBC gives a low ratio — while iron deficiency produces comparatively few small cells, giving a higher ratio for the same MCV.
The index applies only to a microcytic anaemia; run it on a normal or high MCV and the result is not meaningful. It is a screening aid, not a diagnosis, with reported accuracy around 80% against confirmed cases, and it specifically fails where both conditions coexist — a genuinely common situation in populations where thalassaemia trait is prevalent and coexisting iron deficiency is also common, since combined deficiency pulls the ratio toward the middle and away from either characteristic pattern. Every result, in either direction, should be confirmed rather than acted on alone: ferritin to confirm iron deficiency, and haemoglobin electrophoresis or HPLC to confirm thalassaemia trait.
Both bands in this calculator are shown with the same warning tone deliberately. Neither result is a normal finding — the index is only ever applied to a microcytic anaemia that already needs investigating, and the two possible outcomes both point toward a specific confirmatory test rather than toward reassurance.
Frequently asked questions
What does the Mentzer Index tell you?
In a microcytic anaemia, whether beta-thalassaemia trait or iron deficiency is the more likely cause, based on the ratio of MCV to red cell count. It is a screening aid, not a diagnosis.
Why does a low Mentzer Index suggest thalassaemia?
Thalassaemia trait produces many small cells — the red cell count is preserved or raised while the MCV is low, giving a low ratio. Iron deficiency produces fewer small cells overall, giving a higher ratio at the same MCV.
How accurate is the Mentzer Index?
Around 80% in published comparisons. It performs worst where iron deficiency and thalassaemia trait coexist, which is common in populations where thalassaemia is prevalent, since combined deficiency blurs the characteristic pattern of either condition alone.
Does the Mentzer Index apply to a normal MCV?
No. It is only meaningful in a microcytic anaemia. Applying it to a normocytic or macrocytic result is not a valid use of the index and should not guide interpretation.
Related calculators
References
- Mentzer WC Jr. Differentiation of iron deficiency from thalassaemia trait. Lancet. 1973;1(7808):882.
- Hoffbrand AV, Steensma DP. Hoffbrand’s Essential Haematology. 8th ed.