Erythrocyte indices for discriminating thalassaemic and non-thalassaemic microcytosis in Indians.

BACKGROUND: Microcytosis is a common red cell change seen in anaemias of varied aetiology. These include iron deficiency, thalassaemia, chronic disease and sideroblastic anaemias. The microcytosis of heterozygous beta-thalassaemia needs to be distinguished from non-thalassaemic microcytosis for its role in thalassaemia control. Red cell indices derived from automated red cell analysers have been used to discriminate between microcytic patients with a high probability of thalassaemia minor from those with a low probability. There is a controversy on the choice of red cell indices to be used and the cut-off values for this distinction, because the prevalence of iron deficiency as a cause of non-thalassaemic microcytosis is variable. Since no Indian study using receiver operator characteristic (ROC) curves was available to determine the above, we conducted this study.

METHODS: Red cell indices (mean corpuscular volume, total red blood cell count, red cell distribution width, linear discriminant function), serum iron, total iron binding capacity and haemoglobin A2 were estimated in 640 adults with microcytosis (mean corpuscular volume 80 fl). The ROC curves were plotted in all.

RESULTS: Total red blood cell count was observed to be the most efficient single test followed by linear discriminant function and Bessman index. Mean corpuscular volume had the least efficacy. The cut-off values obtained for the Indian population were mean corpuscular volume < or = 76 fl, total red blood cell count > or = 4.9 x 10(12)/L and red cell distribution width > or = 18% and a positive linear discriminant function. These were different from those observed in the West, possibly because of the high prevalence of iron deficiency in India.

CONCLUSION: In countries with a high prevalence of iron deficiency, cut-off values for red cell indices should be recalculated using ROC curves.