Several laboratory tests have been proposed for the differentiation of beta thalassemia from iron deficiency, including decision functions based on red blood cells indices generated by electronic cell counters. The accuracy of these screening methods was assessed in 192 patients with microcytosis known to be secondary to beta thalassemia minor and 72 patients with iron deficiency. The functions evaluated were: 1) discriminant function of England an Fraser: MCV--(5xHb)--RBC--8.4; 2) ratio of MCH/RBC; 3) ratio of MCV/RBC; 4) ratio of (MCV)2 x MCH and 5) the erythrocyte count. The discriminant function of England and Fraser showed the highs, percentage of correct distinction between iron deficiency and beta thalassemia minor, although diagnosis errors occurred in 10%. Mentzer ratio MCV/RBC detected all cases of beta thalassemia but was a poor index for iron deficiency detection. The tested discriminant function, with exception of the Mentzer ratio, although not sufficiently accurate for definitive diagnosis, appears to be a useful technique in the initial screening of patients with microcytosis.39221-3