Q&A column

Bobbi S. Pritt, MD, MSc
Professor of Laboratory Medicine and Pathology
Chair, Division of Clinical Microbiology
Mayo Clinic, Rochester, Minn.
Chair, CAP Microbiology Committee, on behalf of the committee

Q. When a patient is admitted to our hospital, we collect MRSA nares PCR, MRSA axilla by culture, MRSA groin by culture, and vancomycin-resistant Enterococcus by PCR for infection control purposes. Many surrounding facilities have told us they have removed the axilla and groin cultures, but no references were cited to support removing these procedures. Our facility would like to follow the practices of other hospitals, but our providers would like a reference to cite.

Are there best practices or benchmarks from an infection control and microbiology point of view that would allow us to remove the axilla and groin MRSA screen cultures?

A. Screening certain populations of patients upon admission to a health care facility (also known as active surveillance testing) for asymptomatic colonization with methicillin-resistant Staphylococcus aureus (MRSA) is a common practice at health care facilities and is even mandated by law in some states.¹ This testing—who to perform it on, how often to screen, what methods to use, which anatomical sites to sample, and what intervention should take place for patients who have positive results—is an area of debate.² The goal of this testing is to identify patients who are colonized with MRSA and isolate and/or decolonize them to decrease nosocomial transmission and the incidence of MRSA infection. Another possible use of such screening is to avoid or de-escalate empiric anti-MRSA vancomycin therapy in patients who test negative.³

Multiple methods can be used to perform active surveillance MRSA screening, including culture-based approaches (blood agar and/or chromogenic media), PBP2a detection, full antimicrobial susceptibility testing, and/or mecA gene detection. There are no guidelines or best practice recommendations that specify which method is best. Based on the testing method reported in more than 600 participant responses to the College of American Pathologists’ Methicillin-resistant Staphylococcus aureus Screen, 5 Challenge (MRS5) Survey and the more than 600 participant responses to the MRS5-Molecular Survey, institutions vary widely in their approaches. Variables to consider in test method selection include the sensitivity and specificity of the testing method employed, turnaround time, cost, and integration into laboratory workflow.

A practice recommendation document put forth by the Society for Healthcare Epidemiology of America and Infectious Disease Society of America in 2014 states that the anterior nares is the site of colonization that is most frequently positive in most studies but that no single site will detect all colonized individuals.⁴ Due to this finding and to the relative ease of collection at this anatomic location, the anterior nares has come to be considered the primary site for sampling. Other sites also may be sampled to improve the overall sensitivity of surveillance, but it comes at an additional cost.

One approach for determining whether to sample additional anatomic locations is to examine the lab’s in-house data to see how many patients screen positive by groin and/or axilla culture without a concurrent positive nares PCR result. If the data show that the diagnostic yield from sampling additional anatomic locations is low, then removing them from the lab’s active surveillance testing program can be easily justified. On the other hand, if the data show that culturing samples from the groin and axilla in the lab’s patient population identify a significant number of additional colonized patients, then perhaps multisite screening should be continued. A cost analysis based on the lab’s data may help determine the best approach.⁵

Knowing the baseline rate and monitoring for changes in clinical MRSA infection in your institution as well as analyzing data from active surveillance testing for asymptomatic colonization can help guide the use of testing and infection prevention resources when there are significant changes to either metric. Guidance for implementing a MRSA active surveillance testing program can be found in the appendix of the 2014 SHEA/IDSA practice recommendation document.⁴ The population to be screened, frequency, methods used, and anatomic locations screened ideally should be determined collaboratively by an interdisciplinary group that includes the clinical laboratory and infection prevention team at each institution.

1. Lin MY, Hayden MK, Lyles RD, et al. Regional epidemiology of methicillin-resistant Staphylococcus aureus among adult intensive care unit patients following state-mandated active surveillance. Clin Infect Dis. 2018;66(10):1535–1539.

2. Peterson LR, Diekema DJ. To screen or not to screen for methicillin-resistant Staphylococcus aureus. J Clin Microbiol. 2010;48(3):683–689.

3. 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. Clin Infect Dis. 2020;71(5):1142–1148.

4. Calfee DP, Salgado CD, Milstone AM, et al. Strategies to prevent methicillin-resistant Staphylococcus aureus transmission and infection in acute care hospitals: 2014 update. Infect Control Hosp Epidemiol. 2014;35(7):772–796.

5. Peterson LR, Schora DM. Methicillin-resistant Staphylococcus aureus control in the 21st century: laboratory involvement affecting disease impact and economic benefit from large population studies. J Clin Microbiol. 2016;54(11):2647–2654.

Isabella W. Martin, MD
Assistant Professor
Pathology and Laboratory Medicine
Dartmouth Geisel School of Medicine
Clinical Pathologist
Dartmouth-Hitchcock Medical Center
Lebanon, NH
Member, CAP Microbiology Committee