Host - pathogen - treatment triad: host factors matter most in MRSA bacteremia outcomes [PublishAheadOfPrint]

Background: Previous studies have separately emphasized the importance of host, pathogen and treatment characteristics in determining short term or in-hospital mortality for patients with methicillin resistant Staphylococcus aureus (MRSA) bloodstream infections. Less is known about the relative importance of these factors and their interaction in determining short, medium and long-term mortality.

Methods: This is an observational cohort study where all patients admitted to University of New Mexico Health Sciences Center (UNM HSC) between July 2002 and August 2013 with MRSA positive blood cultures were recorded. We collected patients' demographic and treatment data along with genetic markers of the MRSA isolates. Outcomes of interest were determinants of short (30 days), medium (30-90 days) and long-term (>90 days) mortality.

Results: This study included 273 patients with MRSA bacteremia. Short, medium and long-term mortality were 18.7%, 26.4% and 48% respectively. Thirty-day mortality was influenced by host variables along with host-pathogen interaction characteristics. Pitt bacteremia score, malignancy and healthcare exposure contributed to 30 - 90-day mortality while duration of therapy longer than 4 weeks had a protective effect. Age remained a significant risk factor for greater than 90-day mortality while admission leukocytosis was protective. Infection represented the most frequent cause of death for all 3 time frames and it varied from 72.6% in the first 30 days, 60% for 30-90 days to 35.7% at more than 90 days (p= 0.003).

Conclusion: Host characteristics impact short, medium and long term mortality in MRSA bloodstream infections more than genetic pathogen markers and treatment factors.

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