Development of a novel loop-mediated isothermal amplification assay for rapid detection of Mycobacterium leprae in clinical samples

Abstract

Background: Sensitive and definitive diagnostic tests are required for timely treatment of leprosy and to control its transmission. Aim: In the present study, we report the development of loop-mediated isothermal amplification assay using six primers targeting the RLEP gene sequence uniquely present in Mycobacterium leprae. Methods: Tissue punch samples (n = 50) and slit aspirates (n = 50) from confirmed cases of leprosy (M. leprae positive by quantitative polymerase chain reaction), reporting at the Department of Dermatology, Safdarjung Hospital, New Delhi, were analyzed using newly developed closed tube loop-mediated isothermal amplification assay. The sensitivity and specificity; positive predictive value, negative predictive value and accuracy were calculated using MedCalc statistical software. Results: The loop-mediated isothermal amplification assay specifically amplified M. leprae genomic DNA with an analytical sensitivity of 100 fg. About 47 Out of the 50 quantitative polymerase chain reactions confirmed M. leprae positive tissue samples, 47 were positive by loop-mediated isothermal amplification assay (sensitivity 94%; 95% confidence interval 83.5%–98.8%) while only 31/50 were positive by histopathology (sensitivity 62%; 95% confidence interval 47.2%–75.4%). Using slit aspirate samples of these 50 patients, 42 were positive by both quantitative polymerase chain reaction and loop-mediated isothermal amplification assay (sensitivity 84%; 95% confidence interval 70.9%–92.8%) while only 23/50 (sensitivity 46%; 95% confidence interval 31.8%–60.7%) were positive by microscopy. Limitations: In the present study, the leprosy patient cohort was not uniform, as it comprised of a lower number of paucibacillary cases (22%) compared to multibacillary (78%) cases. Conclusion: Loop-mediated isothermal amplification assay established here provides a rapid and accurate diagnostic test for leprosy in terms of sensitivity and specificity. The assay is simple to perform in comparison with other molecular techniques (polymerase chain reaction/quantitative polymerase chain reaction) and has potential for field applicability.