Discordant Xpert® MTB/RIF Ultra rifampicin resistance in Mycobacterium tuberculosis associated with an A451V (Codon 532) rpoB gene mutation.

In a 12-month period 3 Irish-born adults with pulmonary tuberculosis were initially diagnosed by Xpert® MTB/RIF Ultra which detected a rifampicin resistance mutation prompting treatment as potential multidrug resistant cases. Phenotypic drug susceptibility tests determined the isolates to be rifampicin susceptible. Molecular investigations identified an A451V mutation in the Mycobacterium tuberculosis rpoB gene that has not previously been found to cause rifampicin resistance. False positivity should be considered possible with Xpert® MTB/RIF Ultra, particularly in low TB incidence settings.