Extended-Spectrum Beta-Lactamases (ESBLs)

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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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Extended-Spectrum Beta-Lactamases (ESBLs)

  • Revised breakpoints were published by CLSI in 2010.
  • IF the new CLSI interpretive criteria have NOT been placed in use, all confirmed ESBL-producing strains should be reported as resistant for all penicillins, aztreonam, and cephalosporins.
  • IF using the current CLSI interpretive criteria for aztreonam and cephalosporins, the test interpretations for these agents do not need to be changed from sensitive to resistant.
  • CLSI document M100, Table 3A provides the details used to screen and then perform phenotypic confirmation for ESBL-producing Klebsiella pneumoniae, K. oxytoca, Escherichia coli, and Proteus mirabilis. Screening of P. mirabilis for ESBL production should only be performed on clinically relevant isolates, such as those causing bacteremia.
  • Initial screening/phenotypic confirmation can be performed by disk diffusion or broth microdilution methods.