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

Background: Drug 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. Aim: This 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. Methods: A 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. Results: Of 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. Conclusion: Rapid non-conventional and inexpensive methods may serve as a replacement for 1% proportion method in resource limited settings. (C) 2011 Elsevier B.V. All rights reserved.