22.07.2025

How to make use of EUCAST MIC and zone distributions

EUCAST distributions of MIC and Zone diameters (https:/mic.eucast.org; choose either “MIC” or “Disk diffusion”) are freely available. When sufficiently many acceptable distributions are available to the curators, ECOFFs are determined and listed beneath each graph together with confidence intervals. When users have performed 50 – 100 phenotypic tests (MIC determinations or disk diffusion), create your own distribution and check this against the EUCAST reference distribution. Unless the wild type part of the distribution issuperimposableon the EUCAST counterpart, the methodology in the individual laboratory needs reconsideration before EUCAST breakpoints are used. A method which performs well with isolates in the wild type may on occasion perform less well on isolates with resistance mechanisms (bias). Every method needs to be challenged using strains with resistance mechanisms. Some examples of this are listed on the EUCAST Warnings page. Trouble shoot and discuss with your National AST Committee, or contact EUCAST. This text was amended and links added on 2 August, 2025.