Comparison of tests for detection of methicillin-resistant Staphylococcus aureus in a clinical microbiology laboratory.

By microdilution testing, 186 of 1,450 clinical isolates of Staphylococcus aureus were preliminarily classified as oxacillin resistant (MIC > or = 4 micrograms/ml); 15 of these isolates gave conflicting results by alternative methods and were studied further. Only 2 of these (MIC > 4 micrograms/ml) were mecA positive; 13 were inhibited by oxacillin at 4 micrograms/ml. Significant numbers of S. aureus strains classified as resistant with an oxacillin MIC of 4 micrograms/ml may prove susceptible by other methods.