Nasal-Swab Results for Methicillin-Resistant Staphylococcus aureus and Associated Infections

Nasal-swab screening for methicillin-resistant Staphylococcus aureus (MRSA) has a quicker turnaround time than other bacterial culture methods, with results available within 24 h. Although MRSA nasal-swab screening is not intended to guide antimicrobial therapy, this method may give clinicians additional information for earlier tailoring of empiric antimicrobial agents.

To describe the diagnostic characteristics of nasal-swab screening in predicting MRSA infections in hospitalized patients receiving empiric treatment with IV vancomycin.

A retrospective observational chart review was conducted for newly admitted adult patients of the Peter Lougheed Centre in Calgary, Alberta, who were treated empirically with IV vancomycin from January to October 2015 and who underwent nasal-swab screening for MRSA. The diagnostic characteristics of nasal-swab screening were calculated in relation to corresponding culture results for samples collected on admission.

For the 273 patients included in this study, nasal-swab screening for MRSA showed the following diagnostic characteristics in relation to bacterial culture results: sensitivity 58.3% (95% confidence interval [CI] 28.6%-83.5%), specificity 93.9% (95% CI 90.0%-96.3%), positive predictive value 30.4% (95% CI 14.1%-53.0%), negative predictive value 98.0% (95% CI 95.1%-99.3%), positive likelihood ratio 9.5 (95% CI 4.9-18.7), and negative likelihood ratio 0.4 (95% CI 0.2-0.9).

Given the high specificity of this rapid method, clinicians should ensure that patients who are receiving empiric treatment for MRSA infection and who have a positive result on nasal-swab screening continue to receive MRSA coverage until culture results are available. In addition, the high negative predictive value and positive likelihood ratio for nasal-swab screening in a low-prevalence setting suggest that a negative result significantly reduces the probability of MRSA infection. Although nasal-swab screening for MRSA is currently used for determining isolation precautions, this method also had utility in helping clinicians to predict the probability of MRSA infection and in guiding decisions about antimicrobial therapy.