Nitrate Reductase Assay Using Sodium Nitrate for Rapid Drug Susceptibility Testing of Mycobacterium tuberculosis Directly on Sputum Samples

Multidrug-resistant tuberculosis is an increasing public health concern in many parts of the world, especially in lowincome countries, where most cases occur. Traditional drug susceptibility testing is either time-consuming, such the proportion method on solid media, or expensive, such as the BACTEC 960 System. The aim of this study was to evaluate a nitrate reductase assay (NRA) using sodium nitrate (NaNO3) on smear- positive sputum for the detection of multidrugresistant Mycobacterium tuberculosis (MDR-TB) and compared it with the nitrate reductase assay using potassium nitrate (KNO3) and Proportion Method (PM) or Direct Proportion Method (DPM). The NRA-NaNO3 results were compared with other methods for 91 sputum samples for which comparable results were available. The sensitivity (ability to detect true drug resistance) and specificity (ability to detect true drug susceptibility) of the NRA- NaNO3, were 100% and 96%, 93% and 100%, 85% and 98%, and 76% and 97% for Rifampin, Isoniazid, Streptomycin and Ethambutol, respectively. The results were in most cases available in 10 days. NRA-NaNO3 is simple to perform and provides a rapid, accurate, especially in low-income countries and might become alternative to traditional methods.