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Do I have to test Staph aureus with Cefoxitin disks? I use Microscan for MIC's and an Oxacillin screen agar to confirm Methicillin resistance. I do NOT report Oxacillin results on Staph epid or any coag neg Staph. We don't feel it is difficult to read and if that is the only reason to change, must I??
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Dear Jane,
Sorry for the extended delay. There is no legal requirement to use a backup test for detection of oxacillin resistance in staphylococci when using an FDA-cleared device unless the manufacturer of the that device states that this is necessary. This relates to backup tests such as the cefoxitin disk diffusion and oxacillin salt agar screen or tests such as mecA or PBP2a assays. As you know, the cefoxitin disk diffusion test is a surrogate for oxacillin....we still report oxacillin and NOT cefoxitin. Some laboratories prefer a backup test and that is OK. However, if you do not have problems with your routine test system, you should determine if performance of a backup test is the best use of your resources. Some labs use a backup test for oxacillin resistance and others do not. I should add that recently it was noted that the cefoxitin disk diffusion test is more sensitive and specific than the oxacillin salt agar screen in detecting mecA-mediated MRSA. The oxacillin salt agar screen does not work for CoNS.
It would be helpful to report oxacillin on CoNS since about one third are oxacillin susceptible and oxacillin may be used for CoNS infections. When possible, MDs would usually prefer use of a beta-lactam over vancomycin or other drugs in treating serious CoNS infections. Again, the cefoxitin disk is a good test for mecA-mediated resistance in CoNS and as such a surrogate for oxacillin.
Bottom line is that there are options for clinical laboratories. In brief, results for oxacillin should be reported on all staphylococci. Backup testing is not essential but preferred by some labs.
We hope that we were able to answer your question.
Please visit us again at the American Society for Microbiology.
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