Evaluation of nitrate reduction assay, resazurin microtiter assay and microscopic observation drug susceptibility assay for first line antitubercular drug susceptibility testing of clinical isolates of M. tuberculosis

BackgroundDrug resistant tuberculosis (TB) is a growing concern worldwide. Early detection of multidrug-resistant Mycobacterium tuberculosis is of primary importance for both patient management and infection control. Optimal method for identifying drug-resistant M. tuberculosis in a timely and affordable way in resource-limited settings is not yet available.AimThis study evaluated; nitrate reductase assay (NRA), resazurin microtiter assay (REMA) and microscopic observation drug susceptibility assay (MODS) against the conventional 1% proportion method (PM) for the detection of resistance to first line antitubercular drugs, in M. tuberculosis clinical isolates.MethodsA total of one hundred and five clinical isolates of M. tuberculosis; 50 pan sensitive and 55 pan resistant were tested with NRA, REMA and MODS. The 1% proportion method on Lowenstein–Jensen medium was used as reference test.ResultsOf all three methods which were tested NRA was found to be most sensitive and specific. Sensitivity for rifampicin resistance detection was 100%, 94.55% and 92.73% by NRA, REMA and MODS respectively. NRA and REMA were found to be 100% specific, while the MODS was 98% specific for detection of rifampicin resistance. Test results with all these methods were obtained within 8–14 days.ConclusionRapid non-conventional and inexpensive methods may serve as a replacement for 1% proportion method in resource limited settings.

► We evaluated 3 rapid methods NRA, REMA and MODS against conventional 1% PM method.
► These methods are simple, inexpensive, accurate and rapid (results available within 8–14 days).
► Among the 3 methods NRA was most sensitive, specific and accurate.
► These methods may replace 1% PM for rapid detection of MDR-TB in resource limited settings.
► Early detection of drug resistance will help in patient management and support tuberculosis control programs.