By Ralph Tayyar, MD
Infectious Disease Fellow, Stanford University
Dr. Tayyar reports no financial relationships relevant to this field of study.
SYNOPSIS: Nares screening for methicillin-resistant Staphylococcus aureus (MRSA) has a high negative predictive value to rule out MRSA infections at various sites.
SOURCE: Mergenhagen KA, Starr KE, Wattengel BA, et al. Determining the utility of methicillin-resistant Staphylococcus aureus nares screening in antimicrobial stewardship. Clin Infect Dis 2020;71:1142-1148.
Methicillin-resistant Staphylococcus aureus (MRSA) nares screening has been a crucial test in antimicrobial stewardship since it has become essential in deciding on de-escalating anti-MRSA coverage in respiratory infections. Mergenhagen and colleagues looked at the significance of MRSA nares testing in ruling out subsequent MRSA infections at various sites. They retrospectively collected data from patients who were screened for MRSA nares colonization between January 2007 and January 2018 across Veterans Administration (VA) medical centers nationwide.
A total of 561,325 clinical cultures were collected within seven days of nares swabs from 245,833 unique patients. Out of the MRSA nares screened, 73.7% were performed via polymerase chain reaction (PCR) and 26.3% were performed via standard culture techniques. MRSA nares screening was positive in 22.9% of the total screened samples and MRSA was identified in 8.3% of the various clinical cultures.
The study classified clinical cultures per source as follows: blood, intraabdominal, pulmonary, renal, wound, and miscellaneous. For the whole cohort, the negative predictive value (NPV) for isolating MRSA in clinical cultures was 96.9% for MRSA nares screened by PCR and 95.5% for MRSA nares screened by culture. The NPV was lowest in graft cultures at 89.6% and highest in renal system cultures at 99.1%. However, MRSA colonization had a positive predictive value (PPV) as low as 7.6% in predicting MRSA isolation from renal cultures.
The study concluded that a negative MRSA nares screen is a helpful tool in ruling out MRSA infection in various clinical cultures. One could argue that clinicians might feel less comfortable discontinuing empiric MRSA coverage with NPV lower than 99%. However, the large number of samples studied by Mergenhagen and colleagues would give antimicrobial stewardship programs additional arguments for de-escalating empiric MRSA-targeted therapy when appropriate. The study results should be tailored to individualized cases, and the decision regarding screening nares for MRSA should be based on the clinical likelihood of MRSA infections at the different sites and the risk factors of the screened patient.
Moreover, this study had low PPV to the various culture sites and, hence, a positive MRSA nares colonization was not thought to predict the isolation of MRSA. Several other studies have looked at the correlation between MRSA nares testing and non-respiratory infections. One of these studies is a retrospective single-centered cohort in Colorado by Marzec et al that found a 19.89 odds of developing MRSA bacteremia in MRSA nares-colonized patients compared to non-colonizers.1
- Marzec NS, Bessesen MT. Risk and outcomes of methicillin-resistant Staphylococcus aureus (MRSA) bacteremia among patients admitted with and without MRSA nares colonization. Am J Infect Control 2016;44:405-408.