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Interpretation of Oxacillin and Cefoxitin Disk Diffusion Tests

Oxacillin is the agent of choice for standardized MIC methods (broth & agar dilution). However, since 2006, the CLSI has recommended the use of 30 µg cefoxitin disk rather than the oxacillin disk to detect mecA-mediated resistance in the disk diffusion test because the cefoxitin disk test is easier to read and is as sensitive and specific as MIC methods. Results are still reported as "oxacillin-resistant" or "oxacillin-sensitive." Cefoxitin is a better inducer of the mecA gene and gives clearer, easier to read endpoints in disk diffusion tests.
The oxacillin disk is read for light growth within the zone of inhibition using transmitted light (plate held up to light), ANY discernible growth within zone of inhibition is indicative of resistance. The cefoxitin disk is read using reflected light.
Interpretive critieria for cefoxitin disk diffusion test

S.aureus/MRSA<21 mm
> 22 mm