Performance of fusidic acid (CEM-102) susceptibility testing reagents: broth microdilution, disk diffusion, and Etest methods as applied to Staphylococcus aureus.

Fusidic acid (CEM-102) is an established antistaphylococcal agent that has been used in clinical practice for more than 4 decades. The activity of fusidic acid against 778 isolates of Staphylococcus aureus collected from U.S. (53.8% were methicillin-resistant S. aureus [MRSA]) and Canadian (46.5% were MRSA) medical centers was assessed to determine the intermethod accuracy of the Clinical and Laboratory Standards Institute (CLSI) and Etest methods. Broth microdilution MIC results were compared by scattergram analysis to zone diameters around commercially available 5- and 10-microg disks. Acceptable correlation (r = 0.74 to 0.76) was observed for the two disk concentrations, and applying breakpoints of < or = 1 microg/ml (> or = 22 mm) for susceptibility (S) and > or = 4 microg/ml (< or = 19 mm) for resistance (R) provided 99.9% absolute intermethod categorical agreement. Reference CLSI MIC versus Etest MIC results (r = 0.77; 728 strains) showed 55.4% identical results and agreement of 99.7% +/- one log2 dilution. The diagnostic susceptibility testing reagents (including Etest) for fusidic acid (CEM-102) performed at an excellent level of intermethod agreement for the proposed breakpoint criteria.