The page below is a sample from the LabCE course Case Studies in Clinical Microbiology. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Phenotypic Confirmation by Disk Diffusion and Reporting

  • Phenotypic confirmation by disc 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 penicillin's, cephalosporin's, and aztreonam ONLY IF the current CLSI cephalosporin and aztreonam interpretive criteria has NOT been implemented.
  • If using the current criteria, test interpretations do not need to be changed from susceptible to resistant.