Objective To evaluates the value of Mentzer index ( MI ) in differentiating alpha thalassemia carriers from iron deficiency anemia.Methods The data on alpha thalassemia gene,blood cell count,and serum levels of iron and ferritin were retrospectively analyzed in 151 patients with reduced hemoglobin A2( HbA2 )levels on hemoglobin electrophoresis.Based on α -globin gene analysis and levels of iron and ferritin,the patients were assigned to alpha thalassemia carriers group ( n =90 ),or iron deficiency anemia group ( n =61 ).Results MI differed significantly between iron deficiency anemia group and alpha thalassemia carriers with normal iron group ( P ＜ 0.01 ),iron deficiency anemia group and alpha thalassemia carriers with iron deficiency group ( P＜ 0.01 ),iron deficiency anemia group and alpha thalassemia carriers group ( P ＜ 0.01 ); but did not differ significantly between alpha thalassemia carriers with normal iron group and alpha thalassemia carriers with iron deficiency group ( P＞ 0.05 ).The area under the curve ( AUC ) for MI of receiver operating characteristic curve ( ROC ) was 0.849 between iron deficiency anemia group and alpha thalassemia carriers group,and the cut-off point was 13.1.As MI was -- 13.1,the sensitivity for detecting iron deficiency anemia was 96.7％,specificity 61.1％,and Youden index 0.58.If MI combined with RDW,the sensitivity was 100％.Conclusions Mentzer index has certain accuracy in differentiating alpha thalassemia carriers with reduced HbA2 levels from iron deficiency anemia.Both MI and RDW under the cut-off point can exclude the possibility of iron deficiency anemia.