Antimicrobial susceptibility testing with phenotypic methods is based on the measurement of the MIC (mg/L) and breakpoints to categorize bacteria and fungi as susceptible, intermediate or resistant. Phenotypic antimicrobial susceptibility testing requires an agreement on breakpoints and a rigorous standardization of methods and materials. Requirements for defining breakpoints include a definition of doses and dose intervals, information on MIC distributions for the target organisms, definitions of the highest MIC for organisms devoid of phenotypically expressed resistance (the epidemiological cut-off) and information on resistance mechanisms, pharmacokinetics, pharmacodynamics and clinical outcome in trials. In 2001, the breakpoint committees of France, Germany, Norway, Sweden, the Netherlands and the UK were tasked with developing European breakpoints under the umbrella of EUCAST, organized by ESCMID and later also by ECDC. Breakpoints for previously established antibacterial and antifungal agents in Europe have now been harmonized. With the EMA, EUCAST has since 2006 determined breakpoints for new agents. All breakpoints are freely available on the EUCAST web site; these are used in semi-automated antimicrobial susceptibility testing devices and have been employed since 2010 in a EUCAST disc diffusion method. They have been or are now being implemented in most countries inside Europe and many countries outside it. Everything needed to perform and interpret antimicrobial susceptibility testing is freely available from the EUCAST web site, as are aggregated MIC distributions based on more than 26 000 distributions.
Keywords: CLSI; European Committee on Antimicrobial Susceptibility Testing; breakpoints.
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