The Relationship between Vancomycin Minimum Inhibitory Concentrations (MICs) and Treatment Failure among Patients with Methicillin-Resistant Staphylococcus aureus Blood Stream Infections (MRSA-BSI) at the University of Connecticut Health Center (UCHC)
MRSA-BSI has been associated with mortality rates of 20-30%. In recent publications, many factors have been associated with increased risks of treatment failure, including vancomycin MIC >1 μg/ml. Over the past few years, we have observed a substantial increase in the prevalence of MRSA with vancomycin MICs of 2 μg/ml. The purpose of our study is to compare the treatment outcomes of patients with MRSA-BSIs caused by MRSA strains with vancomycin MICs of 1 μg/mL versus 2 μg/mL.
This is an ongoing retrospective observational cohort study of adult patients with MRSA-BSIs that occurred between January 2008-November 2013. Patients were included in the cohort if they were treated with vancomycin for >3 days and had at least 1 serum vancomycin level obtained during therapy. The primary outcome is treatment failure defined as: a composite of 30-day mortality; persistent bacteremia for ≥ 5 days; or clinical nonresponse. Previous published studies using this composite endpoint have reported treatment failure rates of 39% to 53%. Therefore, we used an anticipated difference in treatment failure rate of 25%. An adequate sample size (based on α error of 0.05 and β error of 80%) would be 102 patients in order to achieve adequate power.
Thus far data have been collected and evaluated for a total of 33 patients; vancomycin MICs were 1 μg/mL and 2 μg/mL for 16 and 17 patients, respectively. The median ages were 59 (range 23-92) and 57 years (range 21-85) respectively. There were 11 male and 5 female patients in the MIC ≤1 μg/ml group compared to 8 male and 9 female patients in the MIC ≥2 μg/ml group. Treatment failure was higher in the MIC ≥2 μg/ml group (10/17=64.7%) compared to the MIC ≤1 μg/ml group (6/16=37.5%). Treatment failure was predominantly related to higher numbers of patients with persistent bacteremia in the MIC ≥2 μg/ml group (7/17=41.2%) compared to the MIC ≤1 μg/ml group (3/16=18.8%). There were 6 patients in the MIC ≥2 μg/ml group who required ICU level of care as compared to 3 patients in the MIC ≤1 μg/ml group.
A preliminary analysis of data from 33 patients has already revealed a numerical trend towards higher prevalence of treatment failure driven by persistent bacteremia for the patients who had MRSA-BSIs caused by strains with vancomycin MIC ≥2 μg/ml.
T. A. Gemtessa,
J. R. Aeschlimann, None