Methods: This retrospective cohort study included all adult patients with SAB admitted to a tertiary-care facility from 1/1/2003 to 6/30/2007. Community-associated (CA) SAB was defined as a positive culture collected <48 hours of admission and no healthcare contact. Hospital-associated (HA) SAB was defined as a positive culture collected ≥48 hours of admission. Mortality was defined as in-hospital mortality occurring ≤30 days of culture collection. LOS was the number hospital days after culture collection. Severity of illness was calculated 24 hours before culture collection using the modified Acute Physiology Score. Pulsed-field gel electrophoresis type USA300 was defined as Staphylococcal Protein A type MBQBLO and presence of the arginine catabolic mobile element and Panton-Valentine leukocidin genes. Adjusted Hazard Ratios (HR) and 95% Confidence Intervals (CI) were calculated using Cox proportional hazard models.
Results: Among 814 admissions, 60% had MRSA, and 57% had CA-SAB. 31% of CA-SAB and 10% of HA-SAB isolates were USA300. The median LOS for MRSA patients was 9.6 days and for MSSA patients was 8.3 days (p=0.21). Methicillin resistance was not associated with increased LOS after controlling for severity of illness, intensive care unit (ICU) admission, aggregate comorbidity, year, location of presentation, previous admission, hemodialysis, age, central venous catheter and ventilation, even when stratifying by CA-SAB. Overall 109 (13%) patients died within 30-days of culture collection. For the entire cohort, mortality rates were similar among MRSA (14%) and MSSA (12%; p=0.33). Methicillin resistance was not associated with 30-day mortality among patients with HA-SAB after controlling for potential confounders (HR: 0.68; 95% CI: 0.40, 1.17). However, methicillin resistance was associated with increased hazard of 30-day mortality among patients with CA-SAB after controlling for severity of illness, ICU admission, aggregate comorbidity, year, AIDS and USA300 type (HR: 2.32; 95% CI: 1.16, 4.67).
Conclusions: In HA-SAB, methicillin resistance has little health or economic impact from the hospital perspective. However, in CA-SAB, methicillin resistance was associated with excess mortality. Rapid testing for methicillin resistance could improve patient outcomes and would be especially beneficial for CA-SAB.