Drug susceptibility of Mycobacterium tuberculosis to primary antitubercular drugs by nitrate reductase assay

Background & objectives: Rapid susceptibility testing of Mycobacterium tuberculosis strains is imperative for therapy selection but traditional drug susceptibility tests take weeks or are expensive. In this study we evaluated nitrate reductase assay which utilizes the detection of nitrate reduction as an indication of growth and therefore results can be obtained faster than by visual detection of colonies. Methods: One hundred clinical isolates of M.. tuberculosis were tested for four first line antitubercular drugs by nitrate reductase assay (NRA) and were compared with standard proportion method. The bacteria were inoculated on Lowenstein-Jensen (LJ) medium with primary antitubercular drugs and potassium nitrate was incorporated. After incubation for 714 days, nitrate reduction indicating growth could be detected by colour change when reagents were added. Results: Resistance of isolates as determined by both methods for isoniazid, rifampicin, streptomycin and ethambutol was 32, 35, 62 and 15 per cent respectively. Agreement between NRA and proportion method was 99 per cent for isoniazid and ethambutol. Complete agreement (100%) was found for rifampicin and streptomycin. Results were available in 7-14 days by NRA as compared to proportion method which takes 4-6 wk. Interpretation & conclusion: Nitrate reductase assay is a rapid and inexpensive method for susceptibility testing of M. tuberculosis for primary antitubercular drugs and could be an appropriate alternative to existing methods, particularly in resource-poor settings.