Validation of antibiotic susceptibility testing guidelines in a routine clinical microbiology laboratory exemplifies general key challenges in setting clinical breakpoints.

This study critically evaluated the new European Committee for Antimicrobial Susceptibility Testing (EUCAST) antibiotic susceptibility testing guidelines on the basis of a large set of disk diffusion diameters determined for clinical isolates. We report several paradigmatic problems that illustrate key issues in the selection of clinical susceptibility breakpoints, which are of general importance not only for EUCAST but for all guidelines systems, i.e., (i) the need for species-specific determinations of clinical breakpoints/epidemiological cutoffs (ECOFFs), (ii) problems arising from pooling data from various sources, and (iii) the importance of the antibiotic disk content for separating non-wild-type and wild-type populations.