Abstract: Objective To investigate the possibility of detecting relapse of leprosy early by immuno-serological method. Methods Indirect enzyme-linked immunosorbent assay(ELISA) using natural disaccharide octyl bovine serum albumin (ND-O-BSA) as an antigen(i.e. ND-IgM-ELISA or ND-ELISA) to detect antibody against ND in the sera from normal controls, and to detect the change of antibody level in the sera from cured leprosy and from relapsed leprosy patients. Results A total of 666 cured leprosy patients treated with DDS monotherapy, including 430 cases of multi-bacillary patients(P-MB), 236 cases of pauci-bacillary patients(P-PB), and 15 of them were relapsed, their IgM antibody levels (IgM-AbL) were periodically determined by using above ND-ELISA. The results showed that after DDS monotherapy: ①In MB patients, 95 were Ab positive, 12 of them were diagnosed as relapsed leprosy; 335 cases were Ab negative, only 1 of them was diagnosed as relapsed leprosy; ②In PB patients, 44 were Ab positive and 192 cases Ab negative; 1 case of relapsed leprosy was found; ③The risk of relapse was 6.7 times higher in MB patients than that in PB patients; ④In the group with positive Ab, the cumulative rate of relapse(CRR) was 13.68%, in the group of negative Ab, the CRR was 0.35%, the risk ratio (RR)=36.7(RR>1), attributable risk (AR)=13.33%; ⑤Even though the serum specimens were taken from PB patients, the Ab of majouity of patients converted to positive when relapse occurred. In the majority of patients relapsed the levels of IgM-AbL tended to be increasing or pesistently positive. Usually relapse occurred 1-2 years after IgM-AbL was converted to positive. Relapse occurred 11-30 years after the patients were cured. In very rare case downgrading(from tuberculous to borderline leprosy) occurred. ⑥The levels of IgM-AbL gradually decreased in all relapsed patients after effective treatment except one case whose IgM-AbL was persistently positive. Conclusions The above results indicate that the ND-ELISA might be useful in screening early M.leprae infection and in predicting and monitoring the relapse of leprosy, especially in multibacillary patients.

Key words: Leprosy, Recurrence, Enzyme-linked immunosorbent assay