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

Abstract

Objectives: To evaluate the performance of a cefoxitin 30 μg disc on Iso-Sensitest agar, using a semi-confluent inoculum and overnight incubation at 35-36°C, for detection of methicillin-resistant Staphylococcus aureus (MRSA). Methods: 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 μg disc on Iso-Sensitest agar, using a semi-confluent inoculum and overnight incubation at 35-36°C. This method was compared with the standard SRGA (Swedish Reference Group for Antibiotics) method (oxacillin 1 μg disc on Iso-Sensitest agar supplemented with 5% defibrinated horse blood, confluent growth and 24 h incubation in ambient air at 30°C). Results: The cefoxitin method was excellent, with a sensitivity of 100% and a specificity of 99% using an interpretative zone diameter of S ≥ 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 ≥ 12 mm. Conclusion: 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.