Comparison of Mortality Risk Associated With Bacteremia Due to Methicillin-Resistant and Methicillin-Susceptible Staphylococcus aureus

Abstract

Objective.: To quantify the clinical impact of methicillin-resistance in Staphylococcus aureus causing infection complicated by bacteremia in adult patients, while controlling for the severity of patients' underlying illnesses. Design.: Retrospective cohort study from October 1, 1995, through December 31, 2003. Patients and Setting.: A total of 438 patients with S. aureus infection complicated by bacteremia from a single Veterans Affairs healthcare system. Results.: We found that 193 (44%) of the 438 patients had methicillin-resistant S. aureus (MRSA) infection and 114 (26%) died of causes attributable to S. aureus infection within 90 days after the infection was identified. Patients with MRSA infection had a higher mortality risk, compared with patients with methicillin-susceptible S. aureus (MSSA) infections (relative risk, 1.7 [95% confidence interval, 1.3-2.4]; P < .01), except for patients with pneumonia (relative risk, 0.7 [95% confidence interval, 0.4-1.3]). Patients with MRSA infections were significantly older (P < .01), had more underlying diseases (P = .02), and were more likely to have severe sepsis in response to their infection (P < .01) compared with patients with MSSA bacteremia. Patients who died within 90 days after S. aureus infection was identified were significantly older (P < .01) and more likely to have severe sepsis (P < .01) and pneumonia (P = .01), compared with patients who survived. After adjusting for age as a confounder, comorbidities, and pneumonia as an effect modifier, S. aureus infection-related mortality remained significantly higher in patients with MRSA infection than in those with MSSA infection, among those without pneumonia (hazard ratio, 1.8 [95% confidence interval, 1.2-3.0]); P < .01. Conclusions.: The results of this study suggest that patients with MRSA infections other than pneumonia have a higher mortality risk than patients with MSSA infections other than pneumonia, independent of the severity of patients' underlying illnesses.