Mentzer Index Calculator — Thalassemia vs Iron Deficiency Anaemia

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How this is calculated

Microcytic anaemia (low MCV, typically < 80 fL) has two common causes: iron deficiency and thalassemia trait. They overlap in their blood count findings but require different management — iron supplementation helps iron deficiency but is unnecessary and potentially harmful in thalassemia. The Mentzer Index, proposed by William C. Mentzer Jr. in 1973, offers a simple bedside screen using two values from the routine full blood count: Mean Corpuscular Volume (MCV, in femtolitres) and the Red Blood Cell count (RBC, in 10¹²/L).

In iron deficiency the red cell count tends to fall as the MCV falls — the body makes fewer red cells. In thalassemia trait the red cell count is typically preserved or even elevated despite the small cell size, so the MCV/RBC ratio is lower. A ratio below 13 suggests thalassemia trait; at or above 13 suggests iron deficiency anaemia.

The Mentzer Index is a screening aid with moderate sensitivity and specificity (roughly 70–90% in published studies, varying by population). False positives and negatives are common, especially in combined deficiencies or in populations with a high prevalence of both conditions. A definitive diagnosis requires serum ferritin, serum iron, transferrin saturation, and haemoglobin electrophoresis (HbA2 elevation confirms beta-thalassemia trait). Consult a haematologist for any diagnostic decision.