Evaluation of nitrate reductase assay on Lowenstein–Jensen medium for drug susceptibility testing of Mycobacterium tuberculosis

Abstract

Introduction: Multidrug-resistant tuberculosis is of increasing public health concern and a rapid and inexpensive drug susceptibility test is needed in resource limited settings. Aim: This study aims to compare the efficacy of drug susceptibility testing (DST) of Mycobacterium tuberculosis using nitrate reductase assay (NRA) on solid Lowenstein–Jensen (LJ) medium with proportion method on LJ medium as reference standard. Materials and Methods: From November 2011 to July 2012, 100 isolates of M. tuberculosis were tested for susceptibility to first-line drugs on LJ medium by proportion method and NRA using LJ with potassium nitrate. Results: For various drugs, the sensitivity and specificity by NRA on solid medium were as follows: streptomycin 96.6% and 98.5%; isoniazid 96.87% and 97%; rifampicin 96.3% and 96%; and ethambutol 78.9% and 91.4%, respectively. The concordance for rifampicin and isoniazid between NRA method and LJ proportion method was found to be >95%. Conclusion: NRA was found to be a rapid and sensitive method for DST of M. tuberculosis.