Risk factors and costs associated with methicillin-resistant Staphylococcus aureus bloodstream infections

OBJECTIVES: To compare the cost of hospitalization of patients with methicillin-resistant Staphylococcus aureus (MRSA) bloodstream infection (BSI) versus patients with methicillin-sensitive S. aureus (MSSA) BSI, controlling for severity of underlying illness; and to identify risk factors associated with MRSA BSI. DESIGN: Retrospective case-control study based on medical chart review. SETTING: A 640-bed, tertiary-care hospital in Seattle, Washington. PATIENTS: All patients admitted to the hospital between January 1, 1997, and December 31, 1999, with S. aureus BSI confirmed by culture. RESULTS: Twenty patients with MRSA BSI were compared with 40 patients with MSSA BSI. Univariate analysis identified 5 risk factors associated with MRSA BST. Recent hospital admission (P = .006) and assisted living (P = .004) remained significant in a multivariate model. Costs were significantly higher per patient-day of hospitalization for MRSA BSI than for MSSA BSI ($5,878 vs $2,073; P = .003). When patients were stratified according to severity of illness as measured by the case mix index, a difference of $5,302 per patient-day was found between the two groups for all patients with a case mix index greater than 2 (P < .001). CONCLUSION: These observations suggest that MRSA BSI significantly increases hospitalization costs compared with MSSA BSI, even when controlling for the severity of the patient's underlying illness. As MRSA BSI was also found to be significantly associated with a group of patients who have repeated hospitalizations, such infections contribute substantially to the increasing cost of medical care.