Dynamics of the antibodies in cohorts of cured cases of visceral leishmaniasis: its implication on the validity of serological test, value in prognosis and in post therapeutic assessment

The major disadvantage of a Serological test like Direct Agglutination Test (DAT) for Visceral Leishmaniasis (also called Kala-azar) is its inability to distinguish between recent and past infection. The objective of our study was to look at rate of decline of antibodies in fully cured cases of Kala-azar and length of time it takes for DAT to become negative. Cohort Study involving completely treated Kala-azar cases from Government Hospital during one calendar year of study. Cases were selected on the basis of treatment cohorts 0, 3, 6, 9 & 12 mo after completion of treatment.. Phase I--The cases were traced and after obtaining the informed consent they were subjected to Direct Agglutination Test (DAT). Phase II--The five treatment cohorts, constituting 82 cured cases (average of 15 cured cases per each treatment cohort) were tested again with DAT three months after the first test. The titers of Phase-I and phase-II tests were analyzed for the dynamics of the antibodies for the period. Cutoff-Values of DAT below 1:800 are considered negative. Values of 1:800, 1:1200, 1:1600 and so on are considered positive. The mean titer [Geometric Mean Titer (GMT)] at the start of treatment was 1:1120, which showed steady decline up to six months, plummeting below the cutoff titer for the DAT (1:800) at the ninth month. Antibodies continue to linger for about one year in cured Kala-azar cases even after correct and complete treatment. Single DAT results may be misleading due to high false positivity up to one year after the cure. Paired test defined as two tests 3 mo apart on the same subject. Paired test is highly recommended for diagnosis and prognosis. DAT is still a very useful tool for diagnosis if used along with clinical correlation.