MRSA nares swab is a more accurate predictor of MRSA wound infection compared with clinical risk factors in emergency department patients with skin and soft tissue infections

Objectives Skin and soft tissue infections (SSTI) caused by methicillin-resistant Staphylococcus aureus (MRSA) are prevalent in the emergency department (ED). We determined whether MRSA nasal carriage better identifies patients with MRSA wound infection than clinical risk factors or emergency medicine (EM) provider’s choice of discharge prescriptions. Methods Adult patients presenting to a large academic medical centre ED in the USA with SSTI between May 2010 and November 2011 were screened. Research assistants administered a questionnaire regarding MRSA risk factors, and MRSA nares swab PCR testing, wound culture results and information on antibiotics prescribed at discharge were collected. Measures of classification accuracy for nares swab, individual risk factors and physician’s prescription for MRSA coverage were compared with gold standard wound culture. Results During the study period, 116 patients with SSTI had both wound cultures and nares swabs for MRSA. S. aureus was isolated in 59.5%, most often MRSA (75.4%). Thirty patients (25.9%) had a positive MRSA nares swab and culture for a sensitivity of 57.7% and specificity of 92.2%. Positive predictive value (PPV) for MRSA nares swab was 85.7% and positive likelihood ratio was 7.4, while negative predictive value was 72.8% and negative likelihood ratio 0.5. None of the individual risk factors nor EM provider’s prescription for MRSA coverage had a PPV or positive likelihood ratio higher than nares swabs. Conclusions MRSA nares swab is a more accurate predictor of MRSA wound infection compared with clinical risk factors or EM provider’s choice of antibiotics. MRSA nares swab may be a useful tool in the ED.