Resistance to oxacillin is most accurately determined by testing for mecA or for the protein expressed by mecA, the penicillin-binding protein 2a (PBP 2a, which is also referred to as PBP 2'). Isolates of staphylococci that carry the mecA gene, or that produce PBP 2a should be reported as oxacillin-resistant according to CLSI guidelines.
If MIC tests are performed in addition to disk diffusion, isolates for which oxacillin MICs are > 4µg/mL, are mecA-negative, or PBP 2a negative should be reported as oxacillin-resistant.
Such isolates may have a rare resistance mechanism other than mecA, and may also test susceptible to cefoxitin by disc diffusion. In these scenarios, oxacillin resistance should be reported in accordance with the MIC value.