Utility of MRSA-Positive Nasal Samples in Antimicrobial Stewardship

Nasal screening for methicillin-resistant Staphylococcus aureus (MRSA) has shown high negative predicative values (NPVs) and therefore may be a powerful stewardship tool for de-escalation and avoidance of empirical therapy, according to results of a retrospective cohort study across Veterans Affairs (VA) medical centers in the United State published in Clinical Infectious Diseases.

The study was performed at VA centers nationwide from 2007 to 2018 and the cohort yielded 561,325 clinical cultures from a variety of anatomical sites. Data from patients with positive result for MRSA on nasal screening upon admission or transfer were obtained from the VA Corporate Data Warehouse. Clinical cultures within 7 days of the swab of the nares were evaluated for the presence of MRSA.

Sensitivity for positive MRSA clinical culture was 67.4% and specificity was 81.2%.  The NPV of screening samples ruling out MRSA infection and bloodstream infections were both 96.5%. The NPVwas 98.6% for intra-abdominal cultures, 96.1% for respiratory cultures, 93.1% for wound cultures, and 99.2%, for cultures from the urinary system.

The researchers acknowledged several study limitations, including the fact that some positive cultures may not have been true positives. Also, sterile site was determined based on cultures labels and culture comments that were created via free text and could have introduced misclassification bias. It was also unknown whether each sample collected in the sterile site categories were truly sterile and the methodology may have omitted some true infections taken from non-sterile sites. Patients may have been colonized with MRSA in sites not routinely screened at VA centers, such as the rectum and axilla. Finally, investigators could not determine if a patient was recently decolonized or if they were colonized in areas other than the nares.

Investigators concluded that the results suggested, “a negative MRSA nares swab, taken within 7 days of culture, is useful to predict the absence of MRSA in a subsequent clinical culture.” They cautioned that it is not appropriate to use MRSA-positivity in the nares to predict current MRSA infection. However, the information may be used as a stewardship tool to avoid the use of or de-escalate anti-MRSA therapy and its use may improve patient care by avoiding unnecessary antibiotics and potential nephrotoxicity.

Reference

Mergenhagen KA, Starr KE, Wattengel BA, Lesse AJ, Sumon Z, Sellick JA. Determining the utility of methicillin-resistant Staphylococcus aureus nares screening in antimicrobial stewardship [published online October 1 2019]. Clin Infect Dis. doi:10.1093/cid/ciz974