Evaluation of a cefoxitin 30 μg disc on Iso-Sensitest agar for detection of methicillin-resistant Staphylococcus aureus

To evaluate the performance of a cefoxitin 30 mu g disc on Iso-Sensitest agar, using-a semi-confluent inoculum and overnight incubation at 35-36 degree C, for detection of methicillin-resistant Staphylococcus aureus (MRSA). A total of 457 S. aureus, including 190 MRSA of several defined PFGE types and a number of low-level resistant isolates, were tested with a cefoxitin 30 mu g disc on Iso-Sensitest agar, using a semi-confluent inoculum and overnight incubation at 35-36 degree C. This method was compared with the standard SRGA (Swedish Reference Group for Antibiotics) method (oxacillin 1 mu g disc on Iso-Sensitest agar supplemented with 5% defibrinated horse blood, confluent growth and 24 h incubation in ambient air at 30 degree C). The cefoxitin method was excellent, with a sensitivity of 100% and a specificity of 99% using an interpretative zone diameter of S greater than or equal to 29 mm and R < 29 mm. Its performance was much better than the SRGA method, which with this collection of difficult strains had a sensitivity of only 78% using the current breakpoint of S greater than or equal to 12 mm. We suggest that the cefoxitin method should replace that currently recommended by the SRGA for the detection of MRSA, and that it would fit well into BSAC methodology.