Can anti-PGL-I antibody isotypes differentiate leprosy contacts and leprosy patients?

BACKGROUND: Serological tests for antibody measurement in leprosy have a series of limitations in discriminating contacts and patients. The present paper intends to evaluate if association of more than one antibody isotype in serum samples may be a useful tool in leprosy diagnosis. METHODS: This study evaluated 395 leprosy contacts and 71 leprosy index cases living in endemic municipalities in Northeastern Brazil. The participants were evaluated according to their anti-phenolic glycolipid antigen-I isotype (PGL-I) profile. Serum anti-PGL-I IgM, IgG, and IgA were measured by indirect ELISA. RESULTS: A strong association was found for antibody positivity in MB leprosy index cases. The odds ratios were 6.11 (95% CI 3.08 - 12.16) for IgM, 3.31 (1.66 - 6.61) for IgG, and 16.97 (8.39 - 34.2) for IgA. For IgM associated with one or more isotypes, the OR was 21.0 (95% CI 10.11 - 43.64), and for IgG + IgA, the OR was 17.58 (6.23 - 49.54). The highest diagnostic sensitivity of 76.0% (95% CI 61.8 - 86.9) was observed for IgM, and the lowest value was 24.1% (13.0 - 38.2), which was observed for IgG + IgA isotypes. Regarding presumptive positive predictive values, the lowest value was obtained for IgM at 24.7% (95% CI 18.1 - 32.3), and the highest values were observed for IgM+ one or more isotypes and for IgG + IgA isotype at 60.0% (44.3 - 74.3) and 66.7% (41.0 - 86.7), respectively. CONCLUSIONS: The present work demonstrated that by associating two or more positive antibody isotypes, the risk of facing a real case of leprosy may increase.