Application of the resazurin microtitre assay for the detection of isoniazid and/or rifampicin resistant Mycobacterium tuberculosis clinical isolates in Central Vietnam

Background: Drug resistant tuberculosis (DR-TB) remains a global health problem. The diagnosis, treatment, and management of DR-TB are the major challenges to Vietnam National Tuberculosis Control Program. One of the most important solutions for overcoming this problem is developing reliable and low-cost methods, which have been proposed to Drug Susceptibility Testing (DST) to detect drug resistant tuberculosis. The methods have to be reasonably simple so that they can be widely used in provincial Lung hospitals. Thus, our team conducted this study with the aim to apply the Resazurin Microtiter Assay (REMA) as drug susceptibility testing for detecting rate of phenotyphic isoniazid- (INH) and/or rifampicin- (RIF) resistance of Mycobacterium tuberculosis (MTB) isolates in central Vietnam. Method: A total of 196 Mycobacterium tuberculosis clinical isolates were tested by the REMA and the results were compared with those of BACTEC MGIT 960 system. The REMA was performed in 96-well plates with the concentration of INH and RIF 1.00– 0.031 µg/ml for INH and 2.00– 0.061 µg/ml, respectively. A strain is considered resistant to INH if the MIC is ≥ 0.25 µg/ml. A strain is considered resistant to RIF if the MIC is ≥ 0.5 µg/ml. Results: The REMA results showed 42 (21.42%) MTB isolates resistant to INH, 13(6.63%) MTB isolates resistant to RIF, among them there were 12 (6.61%) isolates resistant to both RIF and INH, which were categorized as multi-drug resistant tuberculosis (MDR-TB). The excellent results of REMA were compared to those of the BACTEC MGIT 960 as the standard method, the sensitivities for isoniazid and rifampicin were 100% for both, the specificities for isoniazid and rifampicin were 99.35%, 98.92%, respectively. The positive predictive value (PPV) and the negative predictive value (NPV) for INH resistance were respectively 100% and 99.49%. The PPV and NPV were respectively 100% and 98.98% for RIF. The accuracies were 99.49% and 98.98% for INH and RIF, respectively. The REMA plate method had the sensitivity of 100% and the specificity of 99.46%, and PPV and NPV of respectively 91.67% and 100% for the identification of MDR-TB strains. Conclusion: Resazurin microtiter assay appears to be a good alternative method for the determination of drug susceptibility testing in low-resource countries such as Vietnam because this method is simple, reliable and inexpensive.