Nine-Hospital Study Comparing Broth Microdilution and Etest Method Results for Vancomycin and Daptomycin against Methicillin-Resistant Staphylococcus aureus

Vancomycin and daptomycin MIC results for 1,800 randomly selected oxacillin (methicillin [meticillin])resistant Staphylococcus aureus (MRSA) bloodstream isolates from nine U. S. hospitals (collected from 2002 to 2006) were determined by a reference broth microdilution (BMD) method using frozen-form panels with precise incremental dilutions and by the Etest technique. The Etest provided vancomycin and daptomycin MIC results that were consistently higher (0.5 to 1.5 log(2) dilution steps) than those provided by the reference BMD method. The dominant MRSA population (91.2% of MRSA isolates) would be categorized as vancomycin nonsusceptible by the MIC results from the Etest method if the susceptibility breakpoint was adjusted downward to <= 1 mu g/ml, as suggested by clinical outcome studies.