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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)

  • There are limitations for ESBL screening tests for E.coli, Klebsiella species, and Proteus mirabilis, so the need exists for confirmatory testing.
  • CLSI suggests confirming these organisms if they are resistant or intermediate to an extended-spectrum cephalosporin.
  • The identification and susceptibility of these isolates should be confirmed if they are not common in your institution. Infection Control should also be consulted to determine if further action or special reporting is necessary.
  • IF the new CLSI interpretive criteria is NOT in place, 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 chephalosporins, the test interpretations for these agents do not need to be changed from sensitive to resistant.
  • CLSI document M100-S26, 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.