Effect of vancomycin minimal inhibitory concentration on the outcome of methicillin-susceptible Staphylococcus aureus...

Staphylococcus aureus endocarditis has a high mortality rate. Vancomycin minimum inhibitory concentration (MIC) has been shown to affect the outcome of methicillin-resistant S. aureus bacteremia, and recent data point to a similar effect on methicillin-susceptible S. aureus bacteremia. We aimed to evaluate the effect of vancomycin MIC on left-sided S. aureus infective endocarditis (IE) treated with cloxacillin.

We analyzed a prospectively collected cohort of patients with IE in a single tertiary-care hospital. Vancomycin, daptomycin, and cloxacillin MIC was determined by E-test. S. aureus strains were categorized as low vancomycin MIC (<1.5 µg/mL) and high vancomycin MIC (≥1.5 µg/mL). The primary endpoint was in-hospital mortality.

We analyzed 93 patients with left-sided IE treated with cloxacillin, of whom 53 (57%) had a vancomycin MIC < 1.5 µg/mL and 40 (43%) a vancomycin MIC ≥ 1.5 µg/mL. In-hospital mortality was 30% (n = 16/53) in patients with a low vancomycin MIC and 53% (n = 21/40) in those with a high vancomycin MIC (P = .03). No correlation was found between oxacillin MIC and vancomycin or daptomycin MIC. Logistic regression analysis showed that higher vancomycin MIC increased in-hospital mortality 3-fold (odds ratio, 3.1; 95% confidence interval, 1.2-8.2) after adjustment for age, year of diagnosis, septic complications, and nonseptic complicated endocarditis.

Our results indicate that vancomycin MIC could be used to identify a subgroup of patients with methicillin-susceptible S. aureus IE at risk of higher mortality. The worse outcome of staphylococcal infections with a higher vancomycin MIC cannot be explained solely by suboptimal pharmacokinetics of antibiotics.