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should testing for both cefoxitin and oxacillin on all staphylococci be routine? How we differentiate between cefoxitin-susceptoble, mec-A negative and oxacillin resistant isolates, if automate susceptibility testing gives you break-point values, not true MIC? Will fox-s, oxa-r isolates grow on oxacillin screen agar?
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Your question is a bit confusing. Cefoxitin is an indicator of oxacillin resistance. For staphylococci, cefoxitin is not reported; only oxacillin is reported. If one tests cefoxitin, it is reported as the oxacillin result. I am not sure why your automated system is testing both drugs, other than having the cefoxitin result double-check the oxacillin result. There should be no discrepancy between the cefoxitin result and the oxacillin result.
The actual MIC is not important; whether the organism is mecA positive is what is important to treatment. The goal of cefoxitin or oxacillin testing is to detect the mecA gene expression. If the gene is expressed, as evidenced by a cefoxitin-resistant result, the organism is oxacillin-resistant. If the cefoxitin is susceptible, the organism is susceptible to oxacillin. An oxacillin screen plate may resolve the discrepancy. However, this test takes 48 h to complete.
It is easier and maybe more accurate to confirm oxacillin-resistance with the cefoxitin disk test with 18 h incubation. Less than or equal to 21 mm is considered resistant for Staphylococcus aureus and S. lugdunensis, while less than or equal to 24 mm is considered resistant for other coagulase-negative staphylococci. Greater zones of inhibition would be considered susceptible to oxacillin. If there is still a confusion about the result, it is good to do a mecA test directly, such as the pbp2a Latex test or a PCR test for mecA. Alternatively, you could send the strain to a reference laboratory that specializes in testing for mecA and for analyzing the rare cases of mecA-negative, oxacillin resistant strains.
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