Microbiology Spectrum (Jan 2024)
Phenotype versus genotype discordant rifampicin susceptibility testing in tuberculosis: implications for a diagnostic accuracy
Abstract
ABSTRACT The World Health Organization recommends culture-based techniques as the gold standard for rifampicin-resistant (RR) tuberculosis. However, as rapid molecular assays are rolled out, discordance in resistance has been observed. The current study compared 516 clinical isolates on Lowenstein–Jenson (LJ) and Mycobacteria Growth Indicator Tube (MGIT) drug susceptibility testings (DSTs) with rpoB sequencing. The discordance rate was higher in MGIT DST (24%) compared to LJ (9.4%). We report discordant results for RpoB mutations 430P (MGIT: 14/20; LJ: 19/20), 435Y (MGIT: 5/21; LJ: 17/21), 445N (MGIT: 11/18; LJ: 15/18), 445L (MGIT: 0/9; LJ: 8/9), 452P (MGIT: 5/20; LJ: 17/20), and 445C (MGIT: 1/6; LJ: 2/6) on LJ and MGIT DSTs, respectively. Mutations N438K and T444A, sensitive on LJ and MGIT DSTs, are novel. Two novel discordant mutations, Q436H+N437del and S428I, were also detected. One novel large deletion (Del) ggaccagaacaacccg/G (Del of DQNNP at positions 435–439) demonstrated concordance on LJ and MGIT DST. Numerous double and triple mutations including some novel combinations, M434V+H445N, L430R+D435Y, H445N+S450W, S428R+L430P, M434L+H445N, D435G+A451V, and M434I+D435G+A451V, have also shown higher discordant results on MGIT DST. The discordance results of RR are higher in MGIT DST compared to LJ. We suggest that the genotypic result should be preferred instead of phenotypic DST in case of discordant mutations. Physicians should be aware of such discordance to prescribe an effective treatment. Untargeted whole-genome sequencing to capture MGIT-sensitive and large deletions may be reconsidered for better treatment outcomes in high-burden countries. IMPORTANCE An accurate diagnosis of drug resistance in clinical isolates is an important step for better treatment outcomes. The current study observed a higher discordance rate of rifampicin resistance on Mycobacteria Growth Indicator Tube (MGIT) drug susceptibility testing (DST) than Lowenstein–Jenson (LJ) DST when compared with the rpoB sequencing. We detected a few novel mutations and their combination in rifampicin resistance isolates that were missed by MGIT DST and may be useful for the better management of tuberculosis (TB) treatment outcomes. Few novel deletions in clinical isolates necessitate the importance of rpoB sequencing in large data sets in geographic-specific locations, especially high-burden countries. We explored the discordance rate on MGIT and LJ, which is important for the clinical management of rifampicin resistance to avoid the mistreatment of drug-resistant TB. Furthermore, MGIT-sensitive isolates may be subjected to molecular methods of diagnosis for further confirmation and treatment options.
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