Performance of Diagnostic Methods for Detection of Drug Resistance Among New Cases of Mycobacterium tuberculosis

OBJECTIVE The increasing incidence of new cases of drug resistant-TB (DR-TB) infection is worrisome, with high rates of new pulmonary morbidities occurring mostly among the vulnerable populations. The performance of diagnostic methods of GeneXpert MTB/RIF assay and phenotypic Drug Susceptibility Test (pDST) for detection of drug resistance among new cases of Mycobacterium tuberculosis (MTB) infections was evaluated. METHODS Sputum samples (n = 546) from newly suspected MTB cases were analysed for phenotypic drug resistance using BACTEC MGIT (B-MGIT) and genotyped with xpert MTB/RIF assay. The predictive performance of B-MGIT culture-based methods and xpert MTB/RIF assay for MTB detection were determined. RESULTS Of the collected sputum samples (n = 546), the highest rates of phenotypic drug resistance (66.7%) were AFB3+ smear positive and MTB with detection rates of 98.2% and 92.7%, respectively, with B-GMIT and Xpert MTB/RIF (p = 0.63). B-MGIT detected higher resistance rates to isoniazide (inh) and rifampicin (rif) of more than 60.0%, and less than 20% resistance to ofloxacin (ofx) and protionamide (proth) compared to Xpert (p < 0.05). B-MGIT had a higher detection rate (98.2%), sensitivity (87.8%), specificity (91.4%), positive predictive value (PPV) (95.8%) (95%CI: 0.778-1.04; p = 0.001) and likelihood of detection of rifampicin resistance (OR:5.76;95%CI:1.01-7.56) compared to Xpert MTB/RIF methods. B-MGIT provided higher AUC-rifampicin of 0.9339 (95%CI: 0.8398 to 1.000) than gene xpert (AUC-rifampicin = 0.9019 [95%CI: 0.7899 to 1.000]). B-MGIT is needed for further confirmation of new cases of MTB identified from acid-fast bacilli (AFB) smear positive sputum. CONCLUSIONS The inclusion of B-MGIT examination of sputum in new TB cases would enhance early detection of drug resistant MTB, mostly from rifampicin negative sputum investigated with GeneXpert and mostly in low resource settings.