Evaluation of the nitrate reductase assay for rapid detection of extensively drug-resistant tuberculosis

BACKGROUND: New diagnostic tools are needed to support tuberculosis (TB) control strategies, particularly in low- and middle-income countries with a high prevalence of TB. OBJECTIVE: To evaluate the nitrate reductase assay (NRA) for the rapid detection of resistance to isoniazid (INH) and rifampicin (RMP), as well as to second-line drugs such as ofloxacin (OFX) and kanamycin (KM). DESIGN: To determine diagnostic accuracy, 192 selected clinical isolates of Mycobacterium tuberculosis were used to compare NRA with BACTEC(TM) 460TB for rapid detection of multidrug-resistant (MDR) and extensively drug-resistant (XDR) TB strains. RESULTS: A good agreement between NRA and the BACTEC 460TB reference method was observed, with good sensitivity and excellent specificity for INH, RMP and OFX. Results for KM were also promising, although the sensitivity for the detection of KM resistance should be improved. CONCLUSION: NRA is a diagnostic tool of promise for the timely detection of M. tuberculosis resistance to first- and second-line drugs. Our study showed a clear potential for the prompt detection of both MDR- and XDR-TB cases. Further studies are needed to optimise the testing of second-line drugs.