Evaluation of direct method of drug susceptibility testing of Mycobacterium tuberculosis to rifampicin and isoniazid by nitrate reductase assay in a national reference laboratory

The aim of this study was to evaluate a simple, rapid, and inexpensive colorimetric nitrate reductase assay (NRA) for direct drug susceptibility testing (DST) of Mycobacterium tuberculosis against rifampicin (RIF) and isoniazid (INH). A total of 118 smear-positive specimens were processed from patients on antituberculosis treatment. A comparison was made between the direct NRA of DST with the direct proportion method and with the internationally accepted indirect 1% proportion method as the “gold standard”. The sensitivity and specificity of the direct NRA and indirect proportion method were 94% and 98%, and 100% and 98% for RIF and INH, respectively. Excellent agreement was found between the 2 tests with κ values of 0.92 and 0.98, and P value was less than 0.001 for RIF and INH. The results in most cases were available in 14 days (turnaround time). The direct NRA is a rapid, accurate, simple, and inexpensive method to determine multidrug resistance from sputum. Direct NRA may become an appropriate alternative method, especially for the resource poor settings.