Impact of vancomycin exposure on outcomes in patients with methicillin-resistant Staphylococcus aureus bacteremia: support for consensus guidelines suggested targets.

BACKGROUND: High rates of vancomycin failure in methicillin-resistant Staphylococcus aureus (MRSA) infections have been increasingly reported over time. The primary objective of our study was to determine the impact of vancomycin exposure and outcomes in patients with MRSA bacteremia initially treated with vancomycin.

METHODS: This was a single-center retrospective analysis of 320 patients with documented MRSA bacteremia initially treated with vancomycin from January 2005 through April 2010. Two methods of susceptibility, Etest and broth microdilution, were performed for all isolates to determine the correlation of susceptibility testing to patient outcomes.

RESULTS: Among a cohort of 320 patients, more than half (52.5%) experienced vancomycin failure. Independent predictors of vancomycin failure in logistic regression included infective endocarditis (adjusted odds ratio [AOR], 4.55; 95% confidence interval [CI], 2.26-9.15), nosocomial-acquired infection (AOR, 2.19; 95% CI, 1.21-3.97), initial vancomycin trough <15 mg/L (AOR, 2.00; 95% CI, 1.25-3.22), and vancomycin minimum inhibitory concentration (MIC) >1 mg/L by Etest (AOR, 1.52; 95% CI, 1.09-2.49). With use of Classification and Regression Tree (CART) analysis, patients with vancomycin area under the curve at 24 h (AUC(24h)) to MIC ratios <421 were found to have significantly higher rates of failure, compared with patients with AUC(24h) to MIC ratios >421 (61.2% vs 48.6%; P = .038).

CONCLUSIONS: In light of the high failure rates associated with this antimicrobial, optimizing the pharmacokinetic/pharmacodynamic properties of vancomycin by targeting higher trough values of 15-20 mg/L and AUC(24h)/MIC ratios ≥400 in selected patients should be considered.