ESBL Test (per CLSI, M100, Table 3A)
Disk:
- Phenotypic confirmation by disk diffusion is performed using either of the previously mentioned antimicrobial agents and testing in combination with clavulanic acid. A ≥5 mm increase between the zone diameters of either agent tested alone, versus testing in combination with clavulanate is taken to be 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 hve NOT been implemented.
- If using the current criteria, test interpretations do not need to be changed from susceptible to resistant.
Broth Microdilution:
- Phenotypic confirmation by broth microdilution is performed using either of the previously mentioned antimicrobial agents and testing in combination with clavulanic acid. A ≥ 3 twofold concentration decrease in an MIC between the agent tested alone, versus testing in combination with clavulanate is taken to be phenotypic confirmation of ESBL production.
- The same reporting should be followed as 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.
