Validation of EUCAST zone diameter breakpoints against reference broth microdilution.

Area of Science:

• Clinical Microbiology
• Antimicrobial Resistance
• Diagnostic Testing

Background:

• The European Committee on Antimicrobial Susceptibility Testing (EUCAST) has harmonized clinical breakpoints in Europe since 2002.
• Development of a disc diffusion method by EUCAST began in 2009, with breakpoints calibrated to MIC values published in December 2009.

Purpose of the Study:

• To validate EUCAST clinical zone diameter breakpoints against the International Standard Organization (ISO) reference broth microdilution method.
• To assess the accuracy and reliability of the EUCAST disc diffusion method for antimicrobial susceptibility testing.

Main Methods:

• A total of 544 bacterial isolates (238 Gram-negative, 306 Gram-positive) were tested.
• Antimicrobial susceptibility testing was performed using both ISO-compliant broth microdilution and EUCAST disc diffusion methodologies.
• Inhibition zone diameters and Minimum Inhibitory Concentration (MIC) values were interpreted using EUCAST clinical breakpoint table version 2.0.

Main Results:

• Overall categorical agreement (CA) between disc diffusion and broth microdilution was high at 97.3% across 5231 organism-agent combinations.
• Low rates of discrepancies were observed: 2.1% minor (mD), 0.5% major (MD), and 0.1% very major (VMD).
• Higher CA was noted for Gram-positive (98.7%) compared to Gram-negative isolates (96.2%). Seven VMDs occurred, primarily in Staphylococcus species and Pseudomonas aeruginosa.

Conclusions:

• The EUCAST disc diffusion method demonstrates excellent correlation with the ISO reference broth microdilution method.
• EUCAST clinical zone diameter breakpoints are reliable for differentiating susceptible, intermediate, and resistant bacterial isolates.
• The validated disc diffusion method supports accurate and standardized antimicrobial susceptibility testing across Europe.