Bacteremia in Staphylococcus aureus pneumonia: outcomes and epidemiology

Staphylococcus aureus represents a major cause of pneumonia in critically ill patients. Although bacteremia may complicate S aureus pneumonia, the epidemiology of and outcomes associated with bacteremia in this syndrome are poorly described.

We retrospectively identified (January 2005-December 2007) all patients admitted to the hospital with S aureus pneumonia necessitating mechanical ventilation. All subjects underwent lower airway and concurrent blood cultures. The prevalence of bacteremia served as a primary end point. We assessed the impact of bacteremia on mortality and length of stay via either logistic regression or a Cox proportional hazard model, respectively. In both models, we controlled for multiple covariates (eg, demographics, severity of illness, comorbidities, and appropriateness of initial antibiotics). We subsequently developed a prediction rule to identify subjects likely to have concurrent bacteremia based on variables assessed at the time of presentation.

The cohort included 59 patients (mean ± SD age, 58.0 ± 17.4 years; 55.9% male, 59.3% methicillin resistant, 39.0% crude mortality). Bacteremia complicated nearly 20% of cases. The mortality rate in those with bacteremia was 39.1% vs 8.3% in persons without bacteremia (P = .007). Three variables were independently associated with mortality in S aureus pneumonia: age, need for vasopressors, and concurrent bacteremia. Bacteremia independently conferred a 6-fold increase in the risk for death (adjusted odds ratio, 5.96; 95% confidence interval [CI], 1.08-33.10). Bacteremia also correlated with a longer length of stay. The adjusted hazard ratio for remaining hospitalized if bacteremic was 2.65 (95% CI, 1.14-6.18). For the clinical prediction rule for concurrent bacteremia, we assigned points as follows: 2 points if the patient had received prior antibiotic therapy and 1 point each for acute lung injury and for the need for vasopressors. As the total score increased, the prevalence of bacteremia increased (P < .001). As a screening test for bacteremia in S aureus pneumonia, the scoring system had good predictive value. The area under the receiver operating curve measured 0.83 (95% CI, 0.72-0.94).

Bacteremia often arises in S aureus pneumonia and is associated with both increased morbidity and mortality. Several simple clinical factors to determine clinical features identify patients with S aureus pneumonia likely to have simultaneous bacteremia.