Evaluation of discrepancies between oxacillin and cefoxitin disk susceptibility for detecting methicillin-resistant Staphylococcus aureus.

The Clinical Laboratory Standards Institute recommends that if both cefoxitin and oxacillin are tested against Staphylococcus aureus and either result is measured as resistant, the organism should be reported as oxacillin resistant. This indicates that discrepancies may be present between oxacillin and cefoxitin sensitivities in S. aureus. In this study, we aimed to investigate the discrepancy between oxacillin and cefoxitin susceptibility in S. aureus clinical isolates. Of 10,980 S. aureus isolates recovered from 2005 to 2010, 27 (0.3%) isolates with discordant results between oxacillin and cefoxitin were collected. Fourteen (oxacillin diameters 10-12 mm) of the 27 strains were susceptible (MICs = 0.5-2 μg/ml) and 13 (6-13 mm) were resistant (4->256 μg/ml) to oxacillin. The cefoxitin MICs of 14 oxacillin-susceptible and 13 oxacillin-resistant strains ranged between 4 and 8 and 8 to 32 μg/ml, respectively. Discrepancies were present between oxacillin and cefoxitin in S. aureus, and these strains should be further tested for oxacillin MICs and for the mecA gene or β-lactamase activity.