Overview of Changes to the Clinical and Laboratory Standards Institute Performance Standards for Antimicrobial Susceptibility Testing, M100, 31st Edition

The Clinical and Laboratory Standards Institute (CLSI) Subcommittee on Antimicrobial Susceptibility Testing (AST) develops and publishes standards and guidelines for AST methods and results interpretation in an annual update to the Performance Standards for Antimicrobial Susceptibility Testing (M100). This minireview will discuss changes to M100 for the 31st edition, including new and revised breakpoints and testing recommendations. New MIC and disk diffusion breakpoints are described for azithromycin (Shigella spp.), imipenem-relebactam (Enterobacterales, Pseudomonas aeruginosa, and anaerobes), and lefamulin (Staphylococcus aureus, Haemophilus influenzae, and Streptococcus pneumoniae), and disk breakpoints are described for azithromycin and Neisseria gonorrhoeae. The rationale behind revised oxacillin MIC breakpoints for select staphylococci is discussed. Updates to test methods include a method for disk diffusion using positive blood culture broth and use of linezolid to predict tedizolid susceptibility. There is clarification on which drugs to suppress on bacteria isolated from the cerebrospinal fluid and clarification on the use of a caret symbol attached to the intermediate category (“I^”) to indicate those antimicrobials that concentrate in the urine.