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 Broth Microdilution and Reporting

  • 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 disc 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.