Detection of Mycobacterium tuberculosis Rifampicin Resistance Conferred by Borderline rpoB Mutations: Xpert MTB/RIF is Superior to Phenotypic Drug Susceptibility Testing

Objective: To compare the ability of detection of borderline rifampicin resistance in Mycobacterium tuberculosis between molecular assay and phenotypic drug susceptibility tests. Methods: Fifty-seven isolates with His445Leu, Asp435Tyr, Leu452Pro, Leu430Pro, His445Asn, Ile491Phe, and His445Ser mutations in rpoB gene identified by whole-genome sequencing conferring borderline rifampicin resistance were included. Molecular-based Xpert MTB/RIF, phenotypic Lowenstein-Jensen (L-J) medium-based drug susceptibility test (DST) with a critical concentration of 40.0 mu g/mL and minimal inhibitory concentration (MIC) assay were performed to detect borderline rifampicin resistance. Results: When using Xpert MTB/RIF, 48/57 (84.2%) isolates exhibited resistance to rifampicin. 25/57 (43.9%) and 33/57 (57.9%) isolates showed rifampicin resistance by L-J medium-based DST with 4 and 6 weeks of incubation, respectively. 30/57 (52.6%) and 40/57 (70.2%) strains were resistant to rifampicin by MIC method at cutoff values of 1.0 and 0.5 mu g/mL, respectively. The detection rate of rifampicin resistance of Xpert MTB/RIF was significantly higher than that of phenotypic methods (p < 0.001). Of the 57 isolates with borderline rpoB mutations, 5 (8.8%) had MICs of 0.25 or 0.12 mu g/mL, 22 (38.6%) had MICs of 0.5 mu g/mL or 1.0 mu g/mL, and 30 (52.6%) other isolates showed MICs >= 2.0 mu g/mL. Conclusion: Molecular-based Xpert MTB/RIF showed superior ability to detect borderline rifampicin resistance over phenotypic DST methods. Extending the incubation time of L-J DST or lowering the cutoff value of the MIC method can improve borderline rifampicin resistance detection.