By Ralph Tayyar, MD

Infectious Disease Fellow, Stanford University

SYNOPSIS: Nares screening for methicillin-resistant Staphylococcus aureus (MRSA) carried 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. It has become essential in deciding on de-escalating anti-MRSA coverage in respiratory infections.

Mergenhagen et al studied 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.

The authors collected 561,325 clinical cultures within seven days of nares swabs from 245,833 unique patients. Out of the MRSA nares screened, 73.7% were performed via 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.

Researchers classified clinical cultures per source as follows: blood, intra-abdominal, 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 carried a positive predictive value (PPV) as low as 7.6% in predicting MRSA isolation from renal cultures.


Mergenhagen et al concluded a negative MRSA nares screen is a helpful tool in ruling out MRSA infection in various clinical cultures. One could argue clinicians might feel less comfortable discontinuing empiric MRSA coverage with NPV lower than 99%.

However, the large number of samples studied 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, there was 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 research groups have studied the correlation between MRSA nares testing and non-respiratory infections. In a retrospective, single-centered cohort in Colorado, a group of investigators found a 19.89 odds of developing MRSA bacteremia in MRSA nares-colonized patients compared to non-colonizers.1


  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.