ABSTRACT
OBJECTIVE: 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. STUDY DESIGN: Comparative study. PLACE AND DURATION OF STUDY: Department of Microbiology, Armed Forces Institute of Pathology, Rawalpindi, from May 2006 to January 2007. PATIENTS AND METHODS: 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. RESULTS: 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. CONCLUSION: 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.
