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.