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Antimicrob. Agents Chemother. doi:10.1128/AAC.00113-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Relationship between Vancomycin MIC and Failure among Patients with MRSA Bacteremia Treated with Vancomycin

Albany College of Pharmacy, Pharmacy Practice Department, Albany, New York; Ordway Research Institute, Albany, New York; Albany Medical Center Hospital, Department of Pathology and Laboratory Medicine, Albany, New York; Albany Medical Center Hospital, Department of Epidemiology, Albany, New York; Albany Medical Center Hospital, Department of Pharmacy, Albany, New York

^* To whom correspondence should be addressed. Email: lodiset{at}acp.edu.

	Abstract

There is a growing concern that vancomycin has diminished activity for MRSA with vancomycin MIC values 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 infection who were treated with vancomycin between 01/2005 and 5/2007. The inclusion criteria were: age 18 years old, non-neutropenic, MRSA culture met CDC criteria for bloodstream infection, received vancomycin therapy within 48 hours of index blood culture, and survived > 24 hours after vancomycin administration. Failure was defined as 30-day mortality, bacteremia 10 days on vancomycin therapy, or 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 value breakpoint derived by CART analysis was 1.5 mg/L. The 66 patients with a vancomycin MIC value 1.5 mg/L had a 2.4-fold increase in failure compared to patients with an MIC value 1.0 mg/L (36.4% and 15.4%, respectively, p=0.049). In the poisson regression, vancomycin MIC value 1.5 mg/L was the independently associated with failure (Adjusted risk ratio 2.7, 95% CI: 1.3- 5.7, p=0.008). These data strongly suggest that patients with MRSA bloodstream infections with vancomycin MIC values 1.5 mg/L respond poorly to vancomycin. Alternative anti-MRSA therapies should be considered for these patients.

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