Background.  Staphylococcus aureus bacteremia (SaB) carries considerable morbidity and mortality. We examined the predictive value of serum concentrations of interleukin (IL)–10, proinflammatory cytokines, and terminal complement on patient survival and SaB duration.

Methods.  Clinical information on consecutive patients with SaB at a tertiary medical center were collected prospectively. Patient serum samples obtained at the day of clinical presentation were assayed for tumor necrosis factor–α, IL-1β, IL-10, and complement membrane attack complex C5b-9 concentrations using enzyme-linked immunoassay. Logistic regression identified predictors of mortality and duration of bacteremia.

Results.  In 59 patients with SaB, 14% died and 17% had prolonged bacteremia (>4 days). Elevated IL-10 serum concentrations (>7.8 pg/mL) identified all 8 patients who died, whereas there were no deaths in patients with normal IL-10 (P = .016). The lack of an IL-1β response (≤0.45 pg/mL) defined all patients with SaB >4 days. In multivariate analysis, patient age (odds ratio [OR], 1.16; P = .022), duration of bacteremia (OR, 1.16; P = .031), and serum IL-10 (OR, 1.05; P = .014) were identified as independent predictors of patient mortality.

Conclusions.  SaB mortality was confined strictly to patients with elevated IL-10 concentrations. We recommend that future clinical trials of SaB stratify patients according to IL-10 and IL-1β serum concentrations in order to better evaluate the impact of therapeutic interventions on patient outcome.