Comparison of two different sensitivity testing agars for detecting methicillin resistance in Staphylococcus aureus

To compare the accuracy of Mueller-Hinton agar and Isosensitest agar using cefoxitin disc for detecting methicillin resistant Staphylococcus aureus using mecA gene PCR assay as gold standard. Comparative study. Department of Microbiology, Armed Forces Institute of Pathology, Rawalpindi, from May 2006 to January 2007. One hundred clinical isolates of Staphylococcus aureus were evaluated; 64 MRSA (methicillin resistant Staphylococcus aureus) and 36 MSSA (methicillin sensitive Staphylococcus aureus) by mecA PCR assay. All the isolates were tested with cefoxitin 30 microg disc using semi-confluent growth on Mueller-Hinton agar as well as on Iso-sensitest agar in ambient air at 35-37 degrees C after an overnight incubation as per recommendations of Clinical and Laboratory Standard Institute. Following diameters provided the best sensitivity and specificity without substantial overlapping between the zones of resistant and sensitive isolates; Mueller-Hinton agar: R < or = 20 mm (sensitivity 100% and specificity 100%), S > or = 22 mm (sensitivity 97.2% and specificity 100%), and Iso-sensitest agar: R < or = 26 mm (sensitivity 100% and specificity 100%), S > or = 26 mm (sensitivity 100% and specificity 100%). High accuracy was obtained with cefoxitin disc on both media. Performance of both media was equally convincing for reliable prediction of methicillin resistance in Staphylococcus aureus by placing cefoxitin 30 microg disc on either of these in routine susceptibility testing.