Epidemiology and outcomes of hospitalizations with MRSA vs MSSA or MRSA+MSSA complicated skin and skin structure infections

Background: Methicillin-resistant Staphylococcus aureus (MRSA) is a frequent cause of complicated skin and skin structure infections (cSSSI). Patients with MRSA have been reported to have worse outcomes when compared to those with methicillin-sensitive Staphylococcus aureus (MSSA). Little is known about the prevalence and implications of co-infection with both MRSA+MSSA.

Objective: To compare current epidemiology and outcomes of patients hospitalized with MRSA cSSSI to those hospitalized with MSSA or MRSA+MSSA.

Methods: We conducted a large multicenter (N=70 hospitals) retrospective cohort study in a US-based database between April 2005 and March 2009. All adult initial admissions with ICD-9-CM codes specific to cSSSI were included. Microbiology results were prioritized to reduce the likelihood of misclassifying colonization as infection, though included patients could have polymicrobial infections. Patients admitted with MRSA, MSSA or MRSA+MSSA were compared with regard to baseline demographic, clinical and hospital characteristics. Unadjusted hospital mortality and length of stay (LOS) among survivors served as primary and secondary outcomes, respectively. 

Results: Of the 7,183 patients with cSSSI, 2,248 (31.3%) had MRSA, 1,885 (26.2%) had MSSA, and 139 (1.9%) had MRSA+MSSA. Patients with MRSA were younger (mean (SD) years) 51.1 (19.4) compared to those with MSSA or MRSA+MSSA (56.3 (19.1) vs. 59.0 (18.8)). MRSA patients were also less likely to be Caucasian (71.4% vs. 74.7% vs. 74.8%), more likely to be on the medical service (74.5% vs. 71.6% vs. 71.9%) and to have a lower comorbidity burden (Charlson comorbidity index = 0: 46.9% vs. 33.5% vs. 28.8%) compared to patients with MSSA or MRSA+MSSA, respectively. Risk factors for healthcare facility-associated infection were common: 32%, 35% and 40% for MRSA, MSSA and MRSA + MSSA respectively. Geographically, MRSA patients were more likely to be in the South, while those co-infected were most prevalent in the Midwest (Graph). Hospital mortality was lowest in the MRSA and highest in the MRSA+MSSA group (2.8% vs. 3.8% vs. 6.5%, p = 0.039). The median LOS among survivors and unknown discharge dispositions was shortest in MRSA and longest in the co-infected group (4.1 vs. 5.2 vs. 7.0 days, p <.0001).    

Conclusions: MRSA represents 53% of all S. aureus cSSSI hospitalizations. Patients hospitalized with MRSA cSSSI are younger, are less likely to be chronically ill, and have lower unadjusted hospital mortality and shorter LOS than those with MSSA or MRSA+MSSA. Those co-infected with MRSA+MSSA, though rare, have worse unadjusted outcomes than those infected with either MRSA or MSSA.  

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Study funded by Astellas Pharma US, Inc, Deerfield, IL