Improvement of a direct agglutination test for field studies of visceral leishmaniasis

To increase the potential for the wide-scale application of our direct agglutination test for visceral leishmaniasis, modifications in the components and procedure were introduced. Supplementation with 0.056 M citrate of the suspension medium stabilized the antigen for 9 weeks at 37°C. To circumvent the need for cooling systems in the field, 0.2% (wt/vol) gelatin was added to the serum diluent instead of fetal bovine serum, with reliable results. Specificity and sensitivity were improved by the incorporation of 0.1 M 2-mercaptoethanol in samples with borderline titers. The test could be performed on samples of whole blood; thus the difficulties of preparation and storage of serum, plasma, or filter paper blood are avoided. For mass screening programs, a single serum dilution of 1:6,400 could be employed, contributing to a further reduction in test expenses. Sera from different geographical areas showed equal reactivities in this direct agglutination test despite the nonhomologous Leishmania donovani antigens used.