245. Importance of Continued ESBL Confirmatory Testing for Enterobacteriaceae
Session: Poster Abstract Session: Diagnostic Microbiology; Antimicrobial Sensitivities
Thursday, October 3, 2013
Room: The Moscone Center: Poster Hall C
Posters
  • IDSA Poster.pdf (59.6 kB)
  • Background: Extended spectrum beta lactamase (ESBL)-producing Enterobacteriaceae are a challenging problem for health care providers. ESBL enzymes inactivate penicillins, aztreonam and cephalosporins, limiting the number of effective antibiotics. Laboratories historically confirmed ESBL (+) organisms using labor intensive manual or automated methods, delaying ESBL identification by 24-48 hr.  In 2010, CLSI revised the interpretative criteria for aztreonam and certain cephalosporins eliminating the need for ESBL confirmatory testing.  We evaluated the 2010 breakpoints for susceptibility reporting of ESBL+ Escherichia coli (E. coli) and Klebsiella pneumoniae (Kpne) isolates.

    Methods: Microscan minimum inhibitory concentration (MIC) results from 10/1/12-12/31/12 were reviewed for 638 E. Coli isolates and 229 ESBL+ Kpne isolates. MIC values were interpreted as sensitive (S), intermediate (I) or resistant (R) based on the 2010 CLSI guidelines for the following antibiotics: aztreonam, cefuroxime, cefotaxime, ceftazidime, ceftriaxone, and cefepime. Interpretations (S, I, R) were statistically analyzed by SPSS software.

    Results: Based on the 2010 guidelines a large percentage (89.03%; 569/638) of ESBL+ E coli strains would be reported as S or I for aztreonam and/or one or more cephalosporins.  Ceftazidime (31.82%) was most frequently reported as either S (22.10%) or I (9.72%).  One hundred seventy (26.65%) isolates had multiple (2-6) drugs incorrectly reported as S or I.

    Further, a large percentage (67.69%; 155/229) of ESBL+ Kpne strains would be reported as S or I for aztreonam and/or one or more cephalosporins.  Cefepime (37.96%) was most frequently reported as either S (29.26%) or I (8.7%).  Twenty-six (11.35%) isolates had multiple (2-6) drugs incorrectly reported as S or I.

    Conclusion: Using the 2010 CLSI guidelines, a significant number of ESBL+ E. Coli and Kpne isolates would be incorrectly reported as S or I to aztreonam and/or other cephalosporins. S/I reporting may relate to strain, resistance mechanism, inoculum effect, or testing variability.  Our data clearly identifies the risk for inappropriate therapy when confirmatory testing is not performed and interpretations are not flagged as R when an ESBL is detected.  This data suggests the need to retain confirmatory ESBL testing despite the new breakpoints.

    Carla Mcwilliams, MD, Division Of Infectious Disease, North Shore Long Island Jewish Health System, Manhasset, NY

    Disclosures:

    C. Mcwilliams, None

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