The usefulness of the nitrate reductase assay for detecting drug-resistant Mycobacterium tuberculosis

Introduction: The early detection of resistance in Mycobacterium tuberculosis is of primary importance for both patient management and infection control. Objective: To evaluate nitrate reductase assay (NRA) performance for the testing of Mycobacterium tuberculosis drug-resistance against first-line anti-tuberculosis drugs, such as rifampicin (RIF), isoniazid (INH), streptomycin (STR) and ethambutol (EMB). Materials and methods: Fifty isolates were tested by using both the proportion method and the nitrate reductase assay. Results: RIF, INH, STR and EMB sensitivity was found to be 92%, 91%, 63% and 80% and 100%, respectively, and a corresponding specificity of 100%, 100%, 100% and 98% by comparing NRA results to those obtained with the gold standard (i.e., the proportion method). The positive predictive values for RIF, INH, STR and EMB were 100%, 100%, 100% and 80% and the negative predictive values were 97%, 93%, 73% and 98%, respectively. The mean time for obtaining results was shorter when using the nitrate reductase assay (10 days) compared to using the proportion method (28 days). Excellent agreement was observed between both phenotypic tests: 98%, 96%, 81% and 96% for RIF, INH, STR and EMB, respectively. Conclusions: The results showed that the nitrate reductase assay is suitable for the early determination of multidrug-resistant tuberculosis (MDR-TB) and is a useful tool for the quick and accurate determination of a rapid M. tuberculosis drug-sensitivity test in countries having low resources.