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We have recently had 2 isolates of E.coli that were reported as ESBL positive by the Vitek 2. However, the Vitek did not change the interpretations for aztreonam, ceftazidime, & cefepime (it reported these as susceptible.) When we repeated the Vitek, we obtained the same results. We do not have any other ESBL confirmatory available. Should we trust the positive Vitek ESBL result, and change the interpretations to resistant manually?
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When using the Vitek 2 ESBL confirmation test for E. coli, K. pneumoniae, or K. oxytoca, if the ESBL Test is positive and AES (Advanced Expert System) recognizes the ESBL phenotype, the CLSI corrections for ESBL (penicillins, cephalosporins, and aztreonam) will be done by AES by therapeutic corrections. These corrections will only appear if you have AES turned on.
If the ESBL phentype is not recognized by AES, but the ESBL Test is positive, not all beta-lactam drugs would be changed to resistant. For example, the AES phenotype "SHV1 hyperproduction" can have either a positive or negative ESBL Test. Results such as this will automatically get flagged for review. It is important to look at the AES phenotypes to see which phenotype match has been made.
I would strongly recommend that you contact your biomerieux representative and discuss your results with them. They may ask for a copy of your lab report to clarify your question and aid in the discussion. In this case, the manufacturer is your best source of information.
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