Interpretation of indeterminate RIF-susceptibility results obtained by rapid molecular diagnostics test

XpertMTB/RIF is a rapid molecular diagnostic tool detecting MTB and RIF-susceptibility within two hours. However, indeterminate RIF-susceptibility results occur and cause uncertainty about clinical judgment. Objective: To evaluate the level of indeterminate RIF-susceptibility results by XpertMTB/RIF, to assess the RIF-susceptibility of such strains and to determine the method that should be used for reference. Methods: Sputum from patients with TB diagnosed in Azerbaijan prisons during 2013-2015 were directly examined by XpertMTB/RIF. Strains with indeterminate RIF-susceptibility results were examined with MGIT-based DST and rpoB sequencing. For comparison, frozen MGIT cultures collected from other TB patients at diagnosis during 2010-2012 were revived and examined with XpertMTB/RIF retrospectively. Results: Of 275 samples examined with XpertMTB/RIF directly and of 232 revived cultures examined retrospectively, 8(2.9%) and 0 had indeterminate RIF-susceptibility results, respectively (p=0.009). MGIT results indicated full concordance with sequencing among the samples showing indeterminate RIF-susceptibility results by XpertMTB/RIF. Conclusion: The indeterminate RIF-susceptibility detected on sputum samples directly by XpertMTB/RIF is significant. However, this drawback may be resolved with the use of Xpert MTB/RIF on cultures. MGIT may be referred to in cases of indeterminate RIF-susceptibility by XpertMTB/RIF.