Rifampicin-resistant TB: discordance between Xpert ^® MTB/RIF and MTBDR <i>plus</i> results

BACKGROUND: South Africa´s diagnostic algorithm for TB diagnosis from 2011 to 2017 employed the Xpert ® MTB/RIF assay as the initial screening test for TB diagnosis and rifampicin (RIF) susceptibility, followed by submission of a specimen for GenoType ® MTBDR plus . This study aimed to determine the concordance between the two assays in terms of RIF susceptibility and explore reasons for discordance. METHODS: This was a retrospective laboratory-based study that included all MTBDR plus results of tests performed at the Braamfontein Mycobacteriology Referral Laboratory between 1 September 2014 and 31 August 2015. The patient´s Xpert RIF result was linked with the MTBDR plus result. RESULTS: The overall concordance between RIF susceptibility results was 96.4%. There were 68 discordant RIF results. The most common reasons for discordance identified were possible false Xpert RIF-resistant results (22%), mixed infection/heteroresistance (16%), transcription errors (7%) and erroneous manual interpretation of the MTBDR plus strip (7%). Xpert RIF resistance detected using delayed hybridisation was associated with discordance. CONCLUSIONS: The overall concordance between the MTBDR plus and Xpert RIF results were very good. Management of discordance should include repeat specimens for Xpert and MTBDR plus and rpo B sequencing. All variables should then be considered before treatment regimens are altered.