Evaluation of the direct agglutination test as an immunodiagnostic tool for kala-azar in India

The direct agglutination test (DAT) has been assessed as a diagnostic procedure for visceral leishmaniasis. Fifty-six of 58 sera (96.5%) from confirmed cases of visceral leishmaniasis, whose bone marrow aspirates contained Leishmania donovani amastigotes, had agglutinating antibodies above the cut-off titre of 1:800. None of the sera from healthy control subjects from non-endemic or endemic areas had anti-leishmanial antibodies. Similarly, none of the sera obtained from cases of malaria or tuberculosis had agglutinating antibodies above the cut-off titre. A significant decline in agglutinating antibody titre in 3 cases following antileishmanial chemotherapy appeared to correlate with regression of clinical symptoms and the absence of amastigotes from bone marrow aspirates. One of 3 cases developed post-kala-azar dermal lesions and sera from this subject had an elevated agglutinating antibody titre. It is concluded that the DAT is a sensitive and specific test to confirm visceral leishmaniasis. As the formalin-fixed promastigotes, stained with Coomassie blue, which are used as antigen could be stored at 4 degrees C for 6 months without any loss of ability to detect anti-leishmanial antibodies, the DAT is recommended for use under field conditions.