Methicillin-resistant Staphylococcus aureus Nares Utility in Non-pneumonia Related Infections

There is an abundance of data to support the use of methicillin-resistant Staphylococcus aureus (MRSA) nasal polymerase chain reaction (PCR) swabs in pneumonia and to guide therapeutic decisions. Conversely, there are few studies evaluating its utility in extrapulmonary infections. The purpose of this study was to assess the utility of this testing in extrapulmonary infections and to determine whether MRSA nasal PCRs can be used as a stewardship tool to guide antimicrobial therapy and treatment via negative predictive values (NPVs) and positive predictive values (PPVs).

This was a retrospective, multicenter, cohort study at a New Jersey health system of patients receiving treatment between March 1, 2023, and July 31, 2023. Data from patients with MRSA nares screening were obtained from electronic medical records. Subsequent clinical cultures collected 7 days before or after the nares swab were evaluated. Sensitivity, specificity, PPVs, and NPVs were assessed for the entire cohort and for subgroups for specific culture sites.

This cohort yielded a total of 742 patients and 1260 clinical cultures from various anatomic sites. Of the patients evaluated, 627 patients had a negative MRSA PCR, and of those with a negative MRSA PCR, 611 patients did not have cultures with MRSA growth. Blood, urine, and wound cultures were the most frequently ordered clinical cultures within our institution, whereas bone/tissue and stool cultures were limited in number. The NPV for blood cultures was 98.7%, urine cultures 100%, wound cultures 92.7%, body fluid/drainage cultures 100%, tissue/bone cultures 100%, and stool cultures 100%. The PPV for blood cultures was 11.1%, urine cultures 1.5%, wound cultures 69.8%, body fluid/drainage cultures 14.3%, tissue/bone cultures 100%, and stool cultures 0%.

Based on the results of this study, the data suggest that a negative MRSA nares PCR obtained within 7 days of a culture has a high NPV for MRSA infections in wound, blood, urine, and body fluid/drainage samples and may be useful to aid in de-escalation of care or discontinuation of antibiotic therapy for certain extrapulmonary infections limited to these sites. Determining predictive values for screening tests is important to establish an antibiotic stewardship tool to guide treatment decisions based on the results. Protocols incorporating MRSA nares PCR could be developed to aid in antimicrobial stewardship and therapeutic management of extrapulmonary infections.