On the Consequences of Poorly Defined Breakpoints for Rifampin Susceptibility Testing of Mycobacterium tuberculosis Complex

Abstract

In a recent report of a systematic review of critical concentrations (CCs), the World Health Organization (WHO) lowered the rifampin (RIF) CC for antimicrobial susceptibility testing (AST) of the Mycobacterium tuberculosis complex using Middlebrook 7H10 medium and the BACTEC Mycobacterial Growth Indicator Tube (MGIT) 960 system from 1 to 0.5 μg/mL. The previous RIF CC for 7H10 had been in use for over half a century. Because it had served as the de facto reference standard, it contributed to the endorsement of inappropriately high CCs for other AST methods, including the U.S. Food and Drug Administration (FDA)-approved MGIT system. Moreover, this resulted in confusion about the interpretation of seven borderline resistance mutations in rpoB (i.e., L430P, D435Y, H445L, H445N, H445S, L452P, and I491F). In this issue of the Journal of Clinical Microbiology, J. Shea, T. A. Halse, D. Kohlerschmidt, P. Lapierre, et al. (J Clin Microbiol 59:e01885-20, 2021, https://doi.org/10.1128/JCM.01885-20) provide evidence that the CC endorsed by the Clinical and Laboratory Standards Institute for the Sensititre MYCOTB system, which is not FDA-approved but is CE-IVD marked in the European Union, is likely also too high. These findings underscore the importance of calibrating AST methods against a rigorously defined reference standard, as recently proposed by the European Committee on Antimicrobial Susceptibility Testing, as well as the value of routine next-generation sequencing for investigating discordant AST results.

Novel and Reported Compensatory Mutations in rpoABC Associate Specifically with Predominant Mycobacterium tuberculosis Rifampicin Resistance Marker rpoB:S450L

“…This was slightly lower than the sensitivity (93.8%, confidence interval 93.3%-94.2%) and specificity (98.2%, confidence interval 98.0%-98.3%) of prediction in a previous study of 27,063 isolates 6 . The lower specificity was likely due to the higher critical concentration use prior to 2021 20 . The high critical concentration has been shown to inconsistently classify RIF R for isolates carrying any of six “borderline” mutations in the RRDR that confer lower level RIF R (Table S2) 6,20,21 .…”

“…The lower specificity was likely due to the higher critical concentration use prior to 2021 20 . The high critical concentration has been shown to inconsistently classify RIF R for isolates carrying any of six “borderline” mutations in the RRDR that confer lower level RIF R (Table S2) 6,20,21 .…”

Molecular characteristics of Mycobacterium tuberculosis drug-resistant isolates from HIV- and HIV+ tuberculosis patients in Russia

“…Some of other mutations revealed in our investigations (Leu511Pro, PR = 4,3% of RIF resistant ClIs from HIV- patients and PR = 3,2% of ClIs from HIV- patients) and rare Asp516Val (4 ClIs from HIV- patients) and Ser522Leu (1 isolate from HIV- patients) were resistant to RIF in our study and were found susceptible in another [ 42 ]. However, according to WHO recommendations any mutation of RRDR, except for synonymous mutations, should be assumed to confer RIF resistance [ 24 , 43 ].…”

Retrospective Diagnostic Accuracy Study of Abbott RealTi m e MTB against Xpert MTB/RIF Ultra and Xpert MTB/RIF for the Diagnosis of Pulmonary Tuberculosis and Susceptibility to Rifampin and Isoniazid Treatment

“…This could have been confirmed by sequencing ( 20 ). Finally, as recent evidence suggests, a critical concentration cutoff of 1.0 μg/ml could also have led to false phenotypic susceptibility ( 21 ).…”