The following is a summary of “Towards clinical breakpoints for non-tuberculous mycobacteria – Determination of epidemiological cut off values for the Mycobacterium avium complex and Mycobacterium abscessus using broth microdilution,” published in the June 2023 issue of the Clinical Microbiology and Infection by Fröberg et al.

Despite their significance for establishing antimicrobial susceptibility testing (AST) breakpoints, minimum inhibitory concentration (MIC) distributions of wild-type isolates have not been systematically evaluated for non-tuberculous mycobacteria (NTM). Researchers collected MIC distributions from 12 laboratories for medications used against Mycobacterium avium complex (MAC) and Mycobacterium abscessus (MAB) obtained by commercial broth microdilution (SLOMYCOI and RAPMYCOI). 

Epidemiological cutoff values (ECOFFs) and tentative ECOFFs (TECOFFs) were determined using EUCAST methodology, which included quality control (QC) strains. The ECOFF of clarithromycin for M. avium was 16 mg/L (n = 1,271). In contrast, the TECOFFs were 8 mg/L for M. intracellulare (n = 415) and 1 mg/L for MAB (n = 1,014), as confirmed by analyzing MAB subspecies without inducible macrolide resistance (n = 235). For amikacin, the ECOFFs for MAC and MAB were 64 mg/L. The WT range for moxifloxacin was >8 mg/L for MAC and MAB. ECOFF and TECOFF values for linezolid were 64 mg/L for M. avium and M. intracellulare, respectively. 

Current CLSI cutoff values for amikacin (16 mg/L), moxifloxacin (1 mg/L), and linezolid (8 mg/L) separated the respective WT distributions. About 95% of MIC values for QC M. avium and M. peregrinum were within the recommended QC ranges. As a first step toward establishing clinical breakpoints for NTM, (T)ECOFFs for multiple antimicrobials against MAC and MAB were defined. Broad wild-type MIC distributions indicate the need for additional method refinement, which is currently being developed by the EUCAST subcommittee on anti-mycobacterial drug susceptibility testing. In addition, we demonstrated that several CLSI NTM breakpoints are inconsistent concerning (T)ECOFFs.

Source: sciencedirect.com/science/article/pii/S1198743X23000605