Improved detection of paucibacillary and multibacillary leprosy using a novel recombinant chimeric protein

The diagnosis of leprosy remains challenging because available diagnostic tests show variable sensitivity and typically require blood sampling. In this study, we assessed the diagnostic performance of the chimeric M2 protein using ELISA assay based on both serum and urine samples from paired specimens of 315 participants. The cohort included paucibacillary (PB; n = 50) and multibacillary (MB; n = 60) leprosy patients, individuals diagnosed with leishmaniasis, Chagas disease, tuberculosis, or HIV infection, as well as household contacts and healthy controls. In serum-based ELISA, mean optical density (OD) values were significantly higher in PB and MB patients (0.544 ± 0.070 and 0.795 ± 0.085, respectively) compared with healthy controls (0.192 ± 0.025). Household contacts exhibited lower OD means (0.130 ± 0.017 for PB contacts and 0.147 ± 0.024 for MB contacts), while samples from patients with other infectious diseases remained below the established cut-off. Receiver operating characteristic curve (ROC) demonstrated sensitivity and specificity exceeding 96.0%, indicating strong discriminatory capacity of the M2 antigen in serum. Urine-based ELISA similarly revealed elevated OD values among PB and MB patients (0.336 ± 0.069 and 0.547 ± 0.076, respectively). In contrast, controls showed a mean OD of 0.170 ± 0.027, and PB and MB household contacts presented lower values (0.108 ± 0.017 and 0.121 ± 0.018, respectively). All other disease groups remained below the cut-off threshold. ROC curve analysis yielded sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) greater than 97.8%, confirming high diagnostic accuracy. Furthermore, M2-specific IgG levels declined substantially following treatment, with reductions of 70.0%-84.0% in MB patients and 55.0%-64.0% in PB patients. Correspondingly, serum and urine OD values decreased from 0.774/0.547 to 0.365/0.243, respectively. Collectively, these findings indicate that the M2 protein represents a promising biomarker for leprosy, demonstrating strong diagnostic performance in both serum and urine assays and supporting its potential utility for diagnosis and treatment monitoring.