Outcome and Predictors of In-hospital 6-week Mortality associated with Invasive Methicillin Resistant Staphylococcus aureus (MRSA) versus Methicillin Sensitive Staphylococcus aureus (MSSA) Infection

Background: Staphylococcus aureus (SA) infections are common and important within the hospital environment. The case fatality rate of invasive Staphylococcus aureus (SA) infections is between 20-40%. Whether the infection is due to methicillin resistant SA (MRSA) or methicillin sensitive SA (MSSA) may determine outcomes. Literature to date is inconclusive regarding whether antimicrobial resistance in SA affects patient outcomes. Host factors, infection-host interactions, and treatment-related factors may also influence case fatality. Objectives: The purpose of this study was to determine if patients with MRSA invasive infections were more likely to die than those with MSSA invasive infections, and what factors were associated with death. Methods: A retrospective matched case control study was designed, comparing cases of MRSA with controls of MSSA invasive disease from hospitals participating in the Canadian Nosocomial Infection Surveillance Program (CNISP). Two analyses were run: the first, to identify the variables associated with MRSA vs. MSSA infections, and the second, to determine the variables associated with death in invasive Staphylococcal aureus (S. aureus) infections. Backward logistic regression analysis was used for the MRSA vs. MSSA analysis and a hierarchical logistic regression model for assessment of risk factors for death. Results: In the logistic regression MRSA model the variables: recent prior use of antibiotics, Charlson Comorbidity Index score > 2 and not having received appropriate empiric antibiotics were associated with MRSA vs. MSSA infections. The hierarchical model identified older age, higher CCI scores, immunosuppression, bloodstream infection, septic shock, neurological dysfunction and not receiving appropriate empiric antibiotic as associated with death. MRSA infection was not more likely to be associated with increased mortality than MSSA infection. Those with a resistant infection (MRSA) however, were less likely to receive appropriate empiric antibiotic treatment. Conclusions: Appropriate empiric antibiotics are the most important and only modifiable risk factor identified. Elderly patients who are on immunosuppressive drugs and have chronic comorbid conditions need to be monitored and screened more often since they are more at risk for death than others.Ph