CLSI publishes new AST standard

Feb. 27, 2020

The Clinical and Laboratory Standards Institute (CLSI) announced the publication of its annually updated antimicrobial susceptibility testing (AST) supplement M100Performance Standards for Antimicrobial Susceptibility Testing, 30th Ed. M100 is available as part of a package with either M02—Performance Standards for Antimicrobial Disk Susceptibility, 13th Ed. or M07— Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically, 11th Ed., or all three documents together.

M02 covers the current recommended methods for disk susceptibility testing and criteria for quality control testing and includes the M02 Quick Guide. M07 covers reference methods for determining minimal inhibitory concentrations of aerobic bacteria by broth macrodilution, broth microdilution and agar dilution. M100 includes annually updated tables for M02, M07 and M11.

Romney Humphries, PhD, D(ABMM) member of the CLSI Subcommittee on AST Testing, said of the changes in the 30th Edition, “In the 2020 edition of M100, we’ve taken the important step of setting, for the first time, colistin and polymyxin B breakpoints for the Enterobacterales. These breakpoints address a major global health need, with the emergence of more and more Gram-negative bacteria that are only susceptible to the polymyxin class of antibiotics. However, use of these drugs is not without substantial risk, as they are associated with high toxicity.”

Humphries continued, “As a result, the AST Subcommittee decided to designate only two interpretive categories for colistin: intermediate and resistant. The reason for this is that the clinical data did not support designating a ‘susceptible’ category, which implies probable clinical success. Along with this breakpoint update, in M100, we describe two new methods for colistin testing: a disk broth elution method and an agar dilution method. Together, these two advances will allow laboratories to test and report these drugs.”

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