To discriminate iron deficiency anemia (IDA) from β thalassemia trait (βTT), several indices obtained from modern blood count analyzers have been reported. Discrimination power of seven indices to differentiate between IDA and βTT, such as Green and King Index (GKI), RDW Index (RDWI), Srivastava Index (SRI), Mentzer Index (MI), Shine and Lal Index (SLI), Ehsani Index (EI), and Sirdah Index (SI), were evaluated. These indices were applied on 47 patients with βTT and on 289 patients with IDA, as confirmed by gold standard tests. Sensitivity, specificity, positive and negative predictive values, efficiency, area under receiver-operating characteristics curve (AUC), and Youden's Index (YI) were calculated. GKI and RDWI showed the highest reliability, as they had the largest AUCs (0.919, 0.912, respectively) and Youden's Index (70.4, 74.6, respectively). Conversely, SLI presented a less satisfactory performance (AUC = 0.786 and YI = 6.6). Data taken together suggest the superiority of GKI and RDWI to discriminate between IDA and βTT.