Thalassemia minor: routine erythrocyte measurements and differentiation from iron deficiency

The clinical differentiation of the causes of microcytosis is difficult because of the lack of a method for the diagnosis of alpha thalassemia. A number of laboratory tests have been proposed for the differentiation of alpha thalassemia from iron deficiency, including decision functions based on the red blood cell indices generated by electronic cell counters. The accuracy of these screening methods was assessed in 93 patients with microcytosis known to be secondary to either iron deficiency or beta thalassemia minor and, prospectively, in 26 patients with microcytosis in whom globin chain synthesis ratio was used to diagnose thalassemia. The functions evaluated were: RBC volume distribution curve; osmotic fragility; erythrocyte count; discriminant function = MCV - (5 X Hgb) - RBC - 8.4; ratio of MCH/RBC; ratio of MCV/RBC; and 0.01 X MCH X (MCV)2. A simplified method of measuring anisocytosis using the RBC volume distribution curve was significantly more accurate (P less than 0.01) in distinguishing iron deficiency from thalassemia than any of the other decision functions. Analysis of red blood cell volume distribution, although not sufficiently accurate for definitive diagnosis, appears to be a useful technic in the initial screening of patients with microcytosis and in determining which additional testing should be done.