AST of bacteria

Antimicrobial susceptibility testing

EUCAST antimicrobial susceptibility testing is performed with phenotypic methods, reference broth microdilution or standardised disk diffusion. The basis of all susceptibility testing is the minimum inhibitory concentration (MIC). Clinical MIC breakpoints determine whether the organism is categorised as susceptible at normal dosing (S), susceptible at increased exposure (I) or resistant (R) to the agent in question. Derivative methods are calibrated to the reference broth microdilution method at the responsibility of the manufacturer. However, the clinical microbiological laboratory takes full responsibility for the final susceptibility report.


Users of EUCAST breakpoints should use the EUCAST disk diffusion method or other susceptibility testing systems calibrated to EUCAST breakpoints and terminology in accordance with EUCAST breakpoint tables.


For videos on how to perform disk diffusion testing according to EUCAST - CLICK here!

For more information - CLICK here.

For information on how the IVDR affects users of EUCAST recommendations - CLICK here!


  • Media preparation
    On how to prepare media for MIC and disk testing
  • MIC determination of nonfastidious and fastidious organisms
    Broth microdilution methodology according to ISO and EUCAST
  • Disk diffusion methodology
    Detailed description of the EUCAST disk diffusion test
  • Disk diffusion implementation
    Guidance documents on how to implement the disk diffusion test
  • Breakpoint tables
    Current MIC and zone diameter breakpoint tables
  • Quality Control and QC tables
    Current tables of MIC and zone diameter ranges for quality control strains
  • Strains with well defined susceptibility 
    Strains with resistance mechanisms where MIC-values (and zone diameters) have been well defined using several different broth microdilution panels before and after freeze drying.
  • Calibration and validation
    Presentation of raw data used in the development and calibration of EUCAST disk diffusion breakpoints against broth microdilution MIC-values.
  • Warnings! 
    EUCAST alerts on malfunctioning susceptibility testing material and procedures.
  • Guidance documents
    Guidance notes on specific susceptibility testing issues
  • MIC determination services provided by the EUCAST Development Laboratory.
  • Previous breakpoints and QC tables
    Earlier versions of breakpoint and QC tables