Why can CLSI and EUCAST interpretations differ for the same organism–drug pair?

Why can CLSI and EUCAST interpretations differ for the same organism–drug pair?

• A. They test different organisms.
• B. They use different disk diffusion media.
• C. They always agree.
• D. They may use different breakpoints and criteria.

Answer: (D)

Interpretive criteria for antimicrobial susceptibility are the thresholds that turn a MIC (or a disk diffusion zone) into categories like susceptible, intermediate, or resistant. These thresholds are not universal; two major organizations, CLSI and EUCAST, set them independently. They base their breakpoints on their own analyses of pharmacokinetics/pharmacodynamics, clinical outcomes, and the distribution of bacterial MICs, using datasets and dosing assumptions that may differ. Because of this, the same organism–drug pair can be classified differently depending on which set of breakpoints is used. Differences can also arise from how each organization handles infection-site considerations, dosing regimens, and the timing of guideline updates. In short, distinct breakpoints and criteria mean the interpretations can diverge even for the same test and the same organism–drug combination.

Interpretive criteria for antimicrobial susceptibility are the thresholds that turn a MIC (or a disk diffusion zone) into categories like susceptible, intermediate, or resistant. These thresholds are not universal; two major organizations, CLSI and EUCAST, set them independently. They base their breakpoints on their own analyses of pharmacokinetics/pharmacodynamics, clinical outcomes, and the distribution of bacterial MICs, using datasets and dosing assumptions that may differ. Because of this, the same organism–drug pair can be classified differently depending on which set of breakpoints is used. Differences can also arise from how each organization handles infection-site considerations, dosing regimens, and the timing of guideline updates. In short, distinct breakpoints and criteria mean the interpretations can diverge even for the same test and the same organism–drug combination.