Direct drug susceptibility testing of Mycobacterium tuberculosis against primary anti-TB drugs in northern India.

INTRODUCTION The present study aimed to evaluate a rapid and inexpensive colorimetric nitrate reductase assay (NRA) performed directly on sputum specimens for detection of drug resistance in Mycobacterium tuberculosis (MTB). METHODOLOGY A total of 55 sputum samples were decontaminated and processed by modified Petroff's method.  A part of the resulting suspension was used to perform direct NRA (DNRA) and direct proportion method (DPM) analysis. Of the 55 samples, 45 could be used to compare the two methods. Indirect drug sensitivity testing (DST) was also done for 14 MTB strains. RESULTS Excellent agreement was found between DNRA and DPM testing with K values of 1, 0.91, 0.91, and 1 for RIF, INH, STR and EMB respectively. The sensitivities and specificities of DNRA compared to that of DPM were observed to be 100 and 100%, 100 and 93%, 95 and 96%, 100 and 100 % for RIF, INH, STR, and EMB respectively. Comparing the results of DNRA, DPM and indirect NRA with those of the gold standard indirect PM for 14 MTB strains showed that sensitivities, specificities and percent agreements were 100, 100 and 100% for all four tested drugs. Results for most of the specimens (55.6%) were available in 21 days with DNRA. CONCLUSIONS We have saved valuable time by omitting the pre-isolation step and conclude that DNRA is a rapid, accurate and inexpensive method for direct DST of MTB and may become an appropriate alternative method for the resource limited settings.