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

Methods: Jiangxi, China.

Objective: To evaluate the performance of the direct nitrate reductase assay (D-NRA) for rapid, low-cost detection of multidrug-resistant (MDR-) and extensively drug-resistant tuberculosis (XDR-TB) in high-burden, resource-limited settings.

Methods: A total of 225 smear-positive sputum samples were collected from consecutive drug-resistant TB subjects. Samples were processed at the Province TB Reference Laboratory and tested for susceptibility to rifampicin (RMP), isoniazid (INH), ofloxacin (OFX), kanamycin (KM) and capreomycin (CPM) by D-NRA, using the indirect Löwenstein-Jensen proportion method (LJ-PM) as reference.

Results: Of the 225 smear-positive sputum samples, 214 isolates were identified as Mycobacterium tuberculosis and analysed for further comparison. The sensitivity of the D-NRA in the detection of resistance to RMP, INH, OFX, KM and CPM was respectively 95.1% (97/102), 93.1% (135/145), 97.4% (76/78), 88.9% (40/45) and 90.6% (29/32); specificity was respectively 100% (112/112), 97.1% (67/69), 100% (136/136), 98.8% (167/169) and 96.7% (176/182). The median time to culture positivity was significantly shorter for NRA than for the indirect LJ-PM (14 days vs. 70 days, P < 0.001).

Conclusions: D-NRA showed high sensitivity and specificity in the rapid diagnosis of MDR- and XDR-TB in a high-burden, resource-limited setting.