[Prevalence of Trypanosoma cruzi antibodies in blood donors]

Blood transfusion is the second most common transmission route of Chagas' disease in endemic areas. Discrepancies between the available diagnostic kits are common, which indicates that a single test is not satisfactory. The aim of this work was to study the seroprevalence of anti-Trypanosoma cruzi markers, to compare sensitivity and specificity of the two screening assays currently in use and to confirm the results with a third test. A total of 20,860 volunteer blood donors were studied. Donations were screened with indirect hemagglutination assay (IHA) and enzyme immunoassay (EIA). Repeatedly reactive samples were assayed with an EIA carried out on strips, to which a mixture of T. cruzi antigens was applied as an horizontal line (DB). Sera that reacted in at least two tests were considered positive. A total of 576 samples were reactive for one or both screening tests (2.76%) and 391 of them (1.87%) were confirmed positive. EIA proved to be more sensitive, with no false negative results (100% sensibility), whereas 98 samples (0.47%) were IHA false negative (74.93% sensibility). Specificity for EIA was 99.3% and for IHA 99.8%. In our case, almost 0.9% of donated blood is discarded because of false reactive anti-T. cruzi results; two thirds correspond to false positive EIA and one third to false positive IHA. It is important to note that in our population we have not registered false negative results for EIA but there were false negative IHA. This fact implies that although the first method is less specific, it is much more sensitive. It is important to confirm the screening results in order to avoid unnecessary donor counselling and permit future donations. The DB test employed in our study results a useful alternative for this purpose.