Field evaluation of the direct detection of multidrug resistant Mycobacterium tuberculosis by nitrate reductase assay on 7H11 agar

In this study we evaluated the performance of colorimetric nitrate reductase assay (NRA) on Middlebrook 7H11 agar instead of Lowenstein-Jensen medium for detection of isoniazid (INH) and rifampin (RIF) resistance directly on 114 smear positive sputum specimens and compared the results with direct proportion method on LJ medium. The results of both methods were in 100% agreement for detection of RIF resistance while agreement for INH was 96.4%. The average turnaround time for NRA was 18.6 days and majority of the specimens gave positive results within 21 days. Thus direct NRA testing on smear positive sputum specimens by using 7H11 agar could be used as a fast, reliable and inexpensive method in resource starved settings.