PURPOSE: Methicillin-resistant Staphylococcus aureus (MRSA) remains an important pathogen in intensive care unit (ICU) patients. Concern exists that higher, yet still susceptible range, minimum inhibitory concentrations (MICs) for vancomycin (VAN) for MRSA lead to worse outcomes. We sought to determine the effect of an increased VAN MIC on mortality in ICU subjects with MRSA bacteremia.
METHODS: We retrospectively indentified (Jan 2009- Dec 2010) persons diagnosed with MRSA bacteremia and who were concurrently treated in the ICU. Mortality served as the primary endpoint. VAN MICs for the isolates were determined via an automated antibiotic susceptibly testing system (Phoenix). We examined potential confounders including demographics (age, gender), severity of illness (presence of shock, need for mechanical ventilation (MV)), and infection related characteristics (nosocomial vs. non-nosocomial, infection source, eventual development of endocarditis). We employed logistic regression to determine the independent impact of an MIC of 2 µcg/ml on hospital mortality.
RESULTS: The final cohort included 100 subjects and an MIC of 2 µcg/ml occurred in 6.0%. As the MIC increased from 0.5 µcg/ml to 1.0 µcg/ml to 2.0 µcg/ml, hospital morality rates increased from 0% to 34.4% to 83.3%, respectively (p=0.005). Patients infected with MIC of 2 µcg/ml strains were 10.2 times more likely to die while hospitalized (95% CI: 1.1-90.8). Independent correlates of morality included: Age > 65 years, development of endocarditis, shock, need for MV, and an MIC of 2 µcg/ml. The adjusted odds ratio for death associated with and elevated MIC equaled 20.96 (95% CI: 1.84-238.56, p=0.014).
CONCLUSIONS: Elevated MICs to VAN in MRSA bacteremia among critically ill subjects occur infrequently. However an MIC of 2 µcg/ml, while technically representing susceptibility to VAN, confers an elevated risk for mortality.
CLINICAL IMPLICATIONS: Monitoring and reporting of the MICs of MRSA to VAN may be important in critically ill patients. Alternative agents other than VAN merit evaluation for MRSA bacteremia, particularly in cases where the MIC of MRSA to VAN is 2 µcg/ml.
DISCLOSURE: Andrew Shorr: Consultant fee, speaker bureau, advisory committee, etc.: Andrew SHorr recived consultant fees from Phizer, Theravance, Forrest, Astellas, and Trius., Consultant fee, speaker bureau, advisory committee, etc.: Andrew SHorr is on the speak bureau for Phizer., Grant monies (from industry related sources): Andrew SHorr has received grant money from Phizer and Astellas.
The following authors have nothing to disclose: Christian Woods, Vinay Patel, Anindita Chowdhury
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