A comparison of disc agar diffusion and agar dilution for the detection of methicillin resistant staphylococci

Ninety two consecutively occurring clinical isolates of Staphylococcus aureus and 71 coagulase negative staphylococci (CONS) were tested by disc agar diffusion (DAD) and agar dilution (Mueller Hinton agar with 4% sodium chloride and 6 mcg/ml oxacillin) for methicillin resistance. There were 6 S. aureus and 33 CONS which were resistant to oxacillin. Five discrepancies (CONS) occurred between the two methods, with all indicating susceptibility by DAD and resistance by agar dilution. Four discrepancies were resolved by minimal inhibitory concentration (MIC) tests. In two of the CONS, the MIC agreed with the DAD results, and in the other two, the MIC agreed with the agar dilution results. Results of this study indicate that DAD is an accurate and acceptable method of detecting methicillin resistance in staphylococci (especially S. aureus) at this institution.