ESBL Test (per CLSI M100)
Disk Diffusion:
- Phenotypic confirmation by disk diffusion is performed using either of the previously mentioned antimicrobial agents and testing with clavulanic acid. A ≥5 mm increase between the zone diameters of either agent tested alone versus testing in combination with clavulanate is considered phenotypic confirmation of ESBL production.
- When reporting confirmed ESBL-producing strains, the interpretation should be reported as resistant for all penicillins, cephalosporins, and aztreonam only if the current CLSI cephalosporin and aztreonam interpretive criteria have not been implemented.
- If using the current criteria, test interpretations do not need to be changed from susceptible to resistant.
Broth Microdilution:
- Phenotypic confirmation of ESBL production is performed by broth microdilution using the previously mentioned antimicrobial agents and testing with clavulanic acid. A ≥3 two-fold concentration decrease in a MIC between the agent tested alone and testing in combination with clavulanate is considered a phenotypic confirmation of ESBL production.
- The same reporting should be followed for the phenotypic confirmation by disk diffusion only if the new CLSI cephalosporin and aztreonam interpretive criteria have not been implemented.
- If using the current criteria, test interpretations do not need to be changed from susceptible to resistant.
