Xpert MTB/RIF probe reactivity to non-tuberculosis mycobacteria cultured in MGIT broth samples

The Xpert MTB/RIF assay can simultaneously detect Mycobacterium tuberculosis (MTB) and rifampicin resistance (RR), though the culture-positive Mycobacteria Growth Indicator Tube (MGIT) broth is not a specified sample type. In clinical settings the assay is sometimes used as an ancillary test to identify MTB and RR in MGIT broth samples, however, limited information is available on the reactivity of the probes to non-tuberculosis mycobacteria (NTM) species. The aim of this study was to assess the results of the Xpert MTB/RIF assay using MGIT broth samples positive for NTM. Of 65 NTM samples tested, 48 (74 %) showed probe signals. Probe reactivity and cycle threshold value differed according to the Mycobacterium species. Ten of 11 (91 %) of Mycobacterium intracellulare samples showed positive Probe A signals, and 23 of 29 (79 %) Mycobacterium avium samples showed positive Probe C signals. Additionally, a sample with M. avium and M. intracellulare co-infection tested false-positive for RR MTB. The proportion of NTM samples that tested positive was higher than that reported previously. These findings could help prevent misinterpretation of Xpert MTB/RIF results.