Prevalence of and risk factors for methicillin-resistant Staphylococcus aureus skin and soft tissue infection in a Canadian emergency department.

Objective: We sought to estimate the period prevalence of methicillin-resistant Staphylococcus aureus (MRSA) skin and soft tissue infection (SSTI) and evaluate risk factors for MRSA SSTI in an emergency department (ED) population.

Methods: We carried out a cohort study with a nested case-control design. Patients presenting to our ED with a wound culture and a discharge diagnosis of SSTI between January 2003 and September 2004 were dichotomized as MRSA positive or negative. Fifty patients with MRSA SSTI matched by calendar time to 100 controls with MRSA-negative SSTI had risk factors assessed using multivariate conditional logistic regression.

Results: Period prevalence of MRSA SSTI was 54.8% (95% confidence interval [CI] 50.2%-59.4%). The monthly period prevalence increased from 21% in January 2003 to 68% in September 2004 (p < 0.01). Risk factors for MRSA SSTI were injection drug use (IDU) (odds ratio [OR] 4.6, 95% CI 1.4-16.1), previous MRSA infection and colonization (OR 6.4, 95% CI 2.1-19.8), antibiotics in 8 weeks preceding index visit (OR 2.6, 95% CI 1.2-8.1), diabetes mellitus (OR 4.1, 95% CI 1.4-12.1), abscess (OR 5.6, 95% CI 1.8-17.1) and admission to hospital in previous 12 months (OR 2.6, 95% CI 1.1-11.2).

Conclusion: The period prevalence of MRSA SSTI between January 2003 and September 2004 was 54.8% at our institution. There was a marked increase in the monthly period prevalence from the beginning to the end of the study. Risk factors are IDU, previous MRSA infection and colonization, prescriptions for antibiotics in previous 8 weeks and admission to hospital in the preceding 12 months. On the basis of local prevalence and risk factor patterns, emergency physicians should consider MRSA as a causative agent for SSTI.