Mitigation of Discordant Rifampicin-Susceptibility Results Obtained by Xpert Mycobacterium tuberculosis/Rifampicin and Mycobacterium Growth Indicator Tube

Abstract

Background: Simultaneous use of genotypic and phenotypic diagnostic tools for detection of rifampicin (RIF) susceptibility may yield discrepant results.

Objective: To measure the discordance between the RIF-susceptibility results by Xpert MTB/RIF and Mycobacterium Growth Indicator Tube (MGIT), to evaluate if application of both tests to the same sample affects the discrepancy, and to evaluate treatment outcome in patients with the discordant strains.

Design: Sputa from patients with tuberculosis managed in the penitentiary system of Azerbaijan during 2011-2015 were examined for RIF susceptibility using Xpert MTB/RIF and MGIT. Strains with discrepant results were sequenced.

Results: Of 532 patients included, 6.2% had discordant RIF-susceptibility results. No significant association of the discordant RIF-susceptibility results with application of both tests on one sample versus sequential samples was found. L511P mutation accounted significantly (p = 0.006) for the discrepancy among those RIF resistant on Xpert MTB/RIF, but sensitive on MGIT. No significant association was identified between the outcomes of treatment with the first- or second-line drugs and the presence of any mutation.

Conclusion: The Xpert MTB/RIF and MGIT testing may be used in sequential sputum samples without increase in the RIF-susceptibility discordance rate. L511P mutation significantly accounts for discordant RIF-susceptibility results, but its clinical relevance may be low.

Keywords: Mycobacterium tuberculosis; drug resistance; mutation; rifampin; sputum; treatment outcome.