Serological methods have been used for detecting
infection with Mycobacterium leprae. We have applied a serological test to explore the possibility it could detect a bacterial relapse among patients who have been cured with
chemotherapy. More specifically we used an indirect
enzyme-linked immunosorbant assay (ELISA) using the natural
disaccharide (ND) of the phenolic
glycolipid antigen of M. leprae linked to
bovine serum albumin as
antigen. Antibody levels were measured in sera from normal controls, active
leprosy cases, cured
leprosy patients, and relapsing
leprosy patients. We correlated antibody levels with the type of
leprosy, the bacterial index, and with relapse among cured
leprosy patients. In our hands, the ND-ELISA, when applied to screening for
infection with M. leprae, had excellent sensitivity, specificity, positive and negative predictive values, and both a low false positive rate and a low false negative rate. Antibody levels gradually increased among active patients from the tuberculoid to the lepromatous end of the
leprosy spectrum. There was a year-by-year fall in antibody levels in patients responding to
chemotherapy. Antibody levels and the bacterial index were correlated using the Spearman's rank correlation method. Serial antibody levels were measured in 666
leprosy patients after being cured with
dapsone monotherapy. Over a three year follow up, 95 multibacillary patients became antibody positive and 12 of them had bacterial relapses of their disease. In contrast, among 335 cases that remained antibody negative, only one relapse was seen. Among 44 paucibacillary cured patients who became antibody positive, there was one relapse. There were 192 such patients who remained antibody negative and one relapsed. The risk of relapse is 6.7 times higher among cured multibacillary patients compared to cured paucibacillary patients. Overall, the cumulative relapse rate among antibody positive cases was 13.7%, compared to 0.4% among antibody negative patients. We conclude that the ND-ELISA is a useful tool both for screening for early
infection with M. leprae and for predicting a relapse in cured patients, particularly in cured multibacillary patients.