(588) Retrospective Study on the Impact of Ongoing or Prior IV Vancomycin Therapy on MRSA Nares Test

Introduction: The impact of anti-Staphylococcus aureus antibiotics, such as vancomycin, on methicillin-resistant Staphylococcus aureus (MRSA) detection via nares screening is unclear. MRSA nares screening, integral to institutional protocols, has demonstrated a high negative predictive value (>95%) for MRSA pneumonia exclusion of diagnosis. Protocols often require MRSA nares swabs to be collected prior to antibiotic exposure, with the reasoning that antibiotic exposure may compromise culture/PCR reliability. Treatment guidelines, such as those by IDSA, recommend empiric MRSA coverage with intravenous vancomycin therapy. Oftentimes vancomycin therapy is initiated prior to specimen collection, raising questions about its effect on the positivity rates of MRSA nares swab results. This retrospective study aims to clarify the time point during vancomycin therapy at which the reliability of MRSA screening may be compromised due to vancomycin exposure.

Methods: The MRSA nares screening consists of a sample collected with a nasal swab that is then transferred to a chromogenic medium for identification. MRSA nares screening cultures were collected starting 14 days before vs 14 days after the start of systemic vancomycin treatment. Positivity rates were calculated by dividing the positive MRSA nares swab tests by the total number of tests collected within predetermined time increments. Chi-square tests and t-tests were used to analyze categorical and continuous data, respectively.

Results: A total of 1,996 MRSA nares swab tests were evaluated from January 2017 to November 2023. The positivity rate of MRSA nares swab before vancomycin administration was 7.1%. Beyond the initial 24 hours after vancomycin exposure, the proportion of positive MRSA nares swab results decreased to 5.1% and remained consistently lower than pre-vancomycin exposure.

Conclusions: This retrospective study provides insights into the impact of systemic vancomycin therapy on the MRSA nares screening. These findings may be useful for optimizing infection control and antimicrobial stewardship processes. Specifically, our findings suggest that the positivity of MRSA nares swabs collected beyond the first 24 hours decreased, but further studies need to be performed to determine clinical significance.