Evaluation of the Impact of Universal Mupirocin Decolonization in Intensive Care Units on the Utility of Methicillin-Resistant <i>Staphylococcus aureus</i> PCR Nasal Swab Testing

BACKGROUND: The reliability of methicillin-resistant Staphylococcus aureus (MSRA) polymerase chain reaction (PCR) nasal swab testing in the context of mupirocin nasal decolonization has been questioned. This study investigated whether the negative predictive value (NPV) of MSRA PCR nasal swab testing following mupirocin decolonization was non-inferior to testing prior to decolonization. METHODS: This retrospective cohort study included adult inpatients admitted to intensive care units (ICUs) from February 2023 - August 2024 who underwent both MRSA PCR nasal swab testing and universal mupirocin decolonization. Patients were divided into 3 groups based on timing of PCR testing: prior to receiving any doses of topical mupirocin, no more than 7 days after, or greater than 7 days after a first dose. The primary outcome was the NPV of MRSA PCR nasal swab testing. RESULTS: 1034 patients were included in the primary analysis; 508 (49.1%) had a MRSA PCR nasal swab collected prior to the start of decolonization and 388 (73.8%) and 138 (26.2%) received nasal swabs > 0 to ≤7 days and >7 days after the start of mupirocin decolonization, respectively. The differences between the NPV of MRSA PCR testing in patients with swabs collected prior versus in patients with swabs collected >0 to ≤7 days and >7 days after the start of mupirocin were 0.3% and -0.4% (1-sided 95% CI -0.9% and CI -2.3%), respectively. CONCLUSIONS: The NPV of MRSA PCR nasal swabs is not diminished after mupirocin decolonization. MRSA PCR testing within 7 days of initiation of decolonization is non-inferior to testing prior to decolonization.