Evaluation of nitrate reductase assay in 7H11 agar for diagnosis of multi-drug-resistant tuberculosis in eastern Nepal

BACKGROUND: Emergence of multi-drug-resistant tuberculosis is a serious challenge for successful global tuberculosis control. Early diagnosis of drug-resistant tuberculosis by direct nitrate reductase assay (NRA) aids in appropriate treatment and reduction in disease transmission, particularly in countries with high tuberculosis burden. The aim of this study was to evaluate the performance of NRA for direct detection of resistance to rifampicin and isoniazid in Mycobacterium tuberculosis in laboratories with limited resources. METHODS: Fifty-eight new smear-positive sputum samples were processed as per the guidelines of revised national tuberculosis control program, India. The performance of NRA on middlebrook 7H11 agar was evaluated for detection of rifampicin and isoniazid resistance directly on smear-positive sputum specimens, and the results were compared with conventional proportion method. Sensitivity and specificity of the test were compared with the gold standard proportion method. Mc Nemar chi-square test was used to find out the significant difference between two methods. RESULTS: Direct NRA for detection of rifampicin resistance was 85.7% sensitive and 100% specific, whereas sensitivity and specificity of isoniazid resistance were 87.5% and 100%, respectively. Agreement between NRA and proportion method was 98% for both the drugs. The mean days of drug susceptibility testing results were 19.3 days for NRA and 72 days for conventional proportion method. The results of NRA were available in 21 days for 83% of the samples. CONCLUSIONS: Direct NRA on middlebrook 7H11 medium is a highly sensitive, reliable, and significantly faster method to perform drug susceptibility testing. It has the potential to be implemented for rapid detection of multi-drug-resistant tuberculosis against insufficient resources.