Relationship between Vancomycin MIC and Failure among Patients with Methicillin-Resistant <i>Staphylococcus aureus</i> Bacteremia Treated with Vancomycin

There is growing concern that vancomycin has diminished activity for methicillin-resistant Staphylococcus aureus (MRSA) infections, with vancomycin MICs at the high end of the CLSI susceptibility range. Despite this growing concern, there are limited clinical data to support this notion. To better elucidate this, a retrospective cohort study was conducted among patients with MRSA bloodstream infections who were treated with vancomycin between January 2005 and May 2007. The inclusion criteria were as follows: at least 18 years old, nonneutropenic, with an MRSA culture that met the CDC criteria for bloodstream infection, had received vancomycin therapy within 48 h of the index blood culture, and survived >24 h after vancomycin administration. Failure was defined as 30-day mortality, bacteremia >or=10 days on vancomycin therapy, or a recurrence of MRSA bacteremia within 60 days of vancomycin discontinuation. Classification and regression tree (CART) analysis identified the vancomycin MIC breakpoint associated with an increased probability of failure. During the study period, 92 patients met the inclusion criteria. The vancomycin MIC breakpoint derived by CART analysis was >or=1.5 mg/liter. The 66 patients with vancomycin MICs of >or=1.5 mg/liter had a 2.4-fold increase in failure compared to patients with MICs of <or=1.0 mg/liter (36.4% and 15.4%, respectively; P = 0.049). In the Poisson regression, a vancomycin MIC of >or=1.5 mg/liter was independently associated with failure (adjusted risk ratio, 2.6; 95% confidence interval, 1.3 to 5.4; P = 0.01). These data strongly suggest that patients with MRSA bloodstream infections with vancomycin MICs of >or=1.5 mg/liter respond poorly to vancomycin. Alternative anti-MRSA therapies should be considered for these patients.