Phenotypic susceptibility

The detection of carbapenem and colistin resistance using phenotypic methods has been agreed between the EURL-PH-AMR and ECDC, following discussions with the members of EURGen-Net and the distribution of a questionnaire to national reference laboratories (NRLs) concerning methods and capabilities in the respective participating countries. Many laboratories are restricted to using their regular routine methods - the difference in sensitivity is clear from Table 1 - and it was decided to offer both alternatives - clinical breakpoints and screening cut-off values. It will be important to devise a reporting method that clearly distinguishes between the breakpoints and methods used by individual laboratories. For colistin, the methodology is straightforward - only broth microdilution and the breakpoints listed in Table 2 are accepted.

Table 1 EUCAST breakpoints for Enterobacterales and carbapenems (v 15.0, 2025)

Table 2 EUCAST MIC breakpoints for Enterobacterales and colistin (v 15.0, 2025)

See also EUCAST MIC-distributions of microorganisms

For a complete and yearly updated list of breakpoints, please consult the EUCAST breakpoint table EUCAST clinical colistin breakpoints are identical to the respective ECOFFs (E. coli and K. pneumoniae both 2 mg/L). The sensitivity for detection of colistin resistance is therefore absolute. Provided the phenotypic test method is properly controlled (see EUCAST Antimicrobial susceptibility testing) it is not necessary to confirm the phenotypic result by genome sequencing. However, the latter may be deemed necessary for epidemiologic reasons.