MRSA Screening

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Also known as: MRSA
Formal name: Methicillin Resistant Staphylococcus aureus screening
Related tests: Wound culture

At a Glance

Why Get Tested?

To detect MRSA carrier status; to evaluate the effectiveness of treatment for a MRSA infection

When to Get Tested?

When you have no signs of an active infection and your doctor wants to determine if you are a MRSA carrier; following treatment of a MRSA infection

Sample Required?

Swab of nose; occasionally swab of wound infection site or skin lesion

Test Preparation Needed?

None

The Test Sample

What is being tested?

These tests detect the presence of methicillin resistant Staphylococcus aureus (MRSA). MRSA are strains of Staphylococcus aureus, or "staph," bacteria that are resistant to the antibiotic methicillin and to related "beta lactam" antibiotics such as oxacillin. First appearing in the early 1960s, MRSA cases and outbreaks have been a problem in confined populations such as hospitals, prisons, and nursing homes for decades.

MRSA strains have caused a significant number of severe skin, lung, bone, and heart-related staph infections that have proven difficult to treat and, in some cases, proven fatal. Most hospitals have instituted measures to attempt to eradicate MRSA and to control the spread of MRSA from person to person. This has been a challenge as staph is a common bacterium that colonizes the skin and is found in the nose of about 25-30% of the population. In the past, only about 0.8% of healthy individuals were colonized with MRSA, but in the last decade this has risen to 1.5%, and in select populations the MRSA colonization rates were as high as 9-14%.

Since the 1960s, there have been occasional outbreaks of MRSA outside of the hospital setting, but in the last few years the number of cases has greatly increased and concern among doctors and other health care workers is growing. In the community, MRSA is causing infections in people of all ages, including young, previously healthy people with no apparent risk factors. Investigations of outbreak cases showed that the bacteria were spread in the community by MRSA colonized or infected people through close contact (such as sports or a day care) and through contact with contaminated objects (such as sports equipment, shared towels, razors, etc.). Early signs of MRSA skin infection have been mistaken as a reaction to a spider bite. MRSA may also cause a bacterial pneumonia that develops after a viral respiratory infection such as influenza.

Studies have also shown that community-acquired strains of MRSA are frequently genetically distinct from hospital-acquired strains of MRSA, indicating that they developed separately. Hospital-acquired MRSA has been typically resistant to more antibiotics routinely prescribed to treat skin infections and, in some cases, has proven to be especially virulent, producing toxins and causing an invasive infection. The distinctions between hospital and community strains of MRSA are weakening, however, as people infected and/or colonized with community-acquired strains come into the hospital setting and as hospital patients and health care workers carry hospital-acquired MRSA into the community.

How is the sample collected for testing?

A nasal swab is collected by rotating a swab inside of each nostril. Occasionally, a swab of a wound infection site or of a skin lesion is collected.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

How is it used?

A MRSA screen is a test that looks solely for the presence of MRSA and no other pathogens. It is primarily used to identify the presence of MRSA in a colonized person or to determine if these resistant bacteria remain at a wound site after the person has been treated for a MRSA infection. On a community level, screening may be used to help determine the source of an outbreak. On a national level, special research tests may inform clinicians and researchers about the unique genetic characteristics of the strains of MRSA circulating in the community or health care setting.

The most widely used test to identify MRSA colonization is the culture. This test confirms the presence of the resistant bacteria and allows the organisms to be further characterized, but culture takes time, usually 1 to 2 days. A nasal swab is collected from the nares (nostrils) of an asymptomatic person and cultured (put onto a special nutrient medium, incubated, and then examined for the growth of characteristic MRSA colonies). A swab may be collected from a wound site or skin lesion of a person who has been previously treated for a MRSA infection and cultured similarly.

Some hospitals have instituted measures to control the spread of MRSA by screening those patients they feel are at risk of being colonized with these resistant bacteria (a carrier) or all new admissions to the hospital. When an outbreak of MRSA is under investigation, screening of health care workers, family members, and close contacts may be performed to identify the source of the infection and to help devise a plan to contain these infections. In some settings, such as nursing homes, a large number of people may be screened to evaluate the spread of colonization in a specific population.

Faster methods of MRSA screening by molecular methods have been developed. These new methods test for the mecA gene that confers resistance to the antibiotics methicillin, oxacillin, nafcillin, and dicloxacillin and other similar antibiotics. Molecular tests for MRSA screening have the potential to detect nasal or wound carriage within hours instead of days required by culture. Research is underway to determine the utility of these rapid and more expensive molecular tests.

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When is it ordered?

A MRSA screening test may be ordered when a doctor, hospital, community health department, or researcher wants to evaluate potential MRSA colonization in an individual, their family members, and/or a group of people in the community as the source of a MRSA infection. Specific populations that have close physical contact, such as a soccer team, residents of a nursing home, health care workers, etc., may be tested for MRSA carrier status when an increased number of infections occur within their close group. Occasionally, a person who has been treated for MRSA infection or for MRSA colonization may be screened to determine whether MRSA is still present.

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What does the test result mean?

If either a culture or a molecular test is positive for MRSA, then the person is colonized with MRSA at the site that was tested, which is usually the nares. If a wound site culture of a person treated for MRSA is still positive, then the bacteria are still present. If the nasal screen or wound site culture is negative, then MRSA is either not present or is present in numbers too low to be detected by the test.

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Is there anything else I should know?

Further testing may be performed on MRSA bacteria when they are isolated in a culture. These additional research tests can identify the type and subtype of S. aureus strains. Although the typing may not be used to determine treatment of the patient, it provides information to track the pattern of disease spread of the infection and characterize the toxins and other virulence factors present in the bacteria.

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Common Questions

1.  Can I get MRSA more than once?

Yes, being successfully treated for MRSA colonization does not prevent you from getting it again.

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2.  Can I be colonized with MRSA and not know it?

Yes, those who are carriers are healthy individuals without signs and symptoms of infection.

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3.  How can I protect myself from getting a MRSA infection?

You should wash your hands frequently, avoid sharing personal items, such as towels, soap, or equipment, and always cover an open wound with a protective bandage.

In certain settings, an antimicrobial ointment may be applied to the nares (nostrils) to decolonize a patient with MRSA. By eliminating the nasal colonization, we can protect the patient from infecting their own skin following, for example, cardiac surgery, and prevent spreading it to others.

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used for Current Review

(Updated 2009 September 28). Recognize and Prevent MRSA Infections. Centers for Disease Control and Prevention [On-line information]. Available online at http://www.cdc.gov/Features/MRSAInfections/ through http://www.cdc.gov. Accessed March 2009.

(Updated 2009 June 18). FDA Clears First Quick Test For Drug-Resistant Staph Infections, Test Identifies MRSA Bacterium in Two Hours. U.S. [On-line information]. Available online at http://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm161552.htm through http://www.fda.gov. Accessed March 2009.

Dugdale, D. et. al. (Updated 2009 May 30). MRSA. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/007261.htm. Accessed March 2009.

(2010 January 21). Genetics Used to Track Transmission of MRSA Bacteria. MedlinePlus HealthDay [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/news/fullstory_94379.html through http://www.nlm.nih.gov. Accessed March 2009.

(Updated 2009 July 22) MRSA and the Workplace. National Institute for Occupational Safety and Health [On-line information]. Available online at http://www.cdc.gov/niosh/topics/mrsa/ through http://www.cdc.gov. Accessed March 2009.

Martinez, J. (Updated 2009 December 16). MRSA Skin Infection in Athletes. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/108972-overview through http://emedicine.medscape.com. Accessed March 2009.

Sources Used in Previous Reviews

Harbarth, S. et. al. (2006 February 27). Evaluation of Rapid Screening and Pre-Emptive Contact Isolation for Detecting and Controlling Methicillin-Resistant Staphylococcus Aureus in Critical Care, An Interventional Cohort Study. [On-line information]. Available online at http://www.medscape.com/viewarticle/523530 through http://www.medscape.com.

(2005 February 2). Laboratory Detection of: Oxacillin/Methicillin-resistant Staphylococcus aureus. CDC [On-line information]. Available online at http://www.cdc.gov/ncidod/dhqp/ar_lab_mrsa.html through http://www.cdc.gov.

(2005 February 3). Community-Associated MRSA Information for Clinicians. CDC [On-line information]. Available online at http://www.cdc.gov/ncidod/dhqp/ar_mrsa_ca_clinicians.html through http://www.cdc.gov.

(2006 March 7). The Growing Menace of Community-Acquired Methicillin-Resistant Staphylococcus aureus. Annals of Internal Medicine v144 (5).

Seybold, U. et. al. (2006 March 1). Emergence of Community-Associated Methicillin-Resistant Staphylococcus aureus USA300 Genotype as a Major Cause of Health Care-Associated Blood Stream Infections. CID 2006:42 pp 647-654 [On-line journal].

Fridkin, S. (2005 April 7). Methicillin-Resistant Staphyloccus aureus Disease in Three Communities. N Engl J Med 352;14.

Case, M. (2006 March 15). Fast staph test limits spread, at a high price. Post-gazette.com [On-line information]. Available online at http://www.post-gazette.com/pg/pp/06074/670567.stm through http://www.post-gazette.com.

Peck, P. (2006 February 6). Fast MRSA Test Cuts Infection Rate in Medical ICU. Medpage Today [On-line information]. Available online at http://www.medpagetoday.com/tbprint.cfm?tbid=2623 through http://www.medpagetoday.com.

Waknine, Y. (2006 March 31). Highlights from MMWR: Outbreaks of MRSA Infection in Newborns and More [On-line information]. Available online at http://www.medscape.com/viewarticle/528952 through http://www.medscape.com.

Kuehnert, M. et. al. (2005). Methicillin-Resistant Staphylococcus aureus Hospitalizations, United States. Medscape from Emerg Infect Dis 11 (6) [On-line information]. Available online at http://www.medscape.com/viewarticle/505669 through http://www.medscape.com.

Creech II, C. et. al. (2005 July). Increasing Rates of Nasal Carriage of Methicillin-Resistant Staphylococcus aureus in Health Children. The Pediatric Infectious Disease Journal v24 (7) 617-621.

Bradley, S. (2006 February 22). Staphylococcus aureus Pneumonia: Emergence of MRSA in the Community. Medscape from Semin Respir Crit Care Med [On-line information]. Available online at http://www.medscape.com/viewarticle/521338 through http://www.medscape.com.

(2006). "Search and Destroy" Strategy Can Reduce Nosocomial MRSA Prevalence. Medscape from Reuters Health Information [On-line information]. Available online at http://www.medscape.com/viewarticle/527548 through http://www.medscape.com.

(2006). New Test Diagnoses MRSA in One Day. Medscape from Reuters Health Information [On-line information]. Available online at http://www.medscape.com/viewarticle/523635 through http://www.medscape.com.

Goossens, H. (2006). Antibiotic Resistance: Highlights of the 16th European Congress on Clinical Microbiology and Infectious Diseases. Medscape [On-line information]. Available online at http://www.medscape.com/viewarticle/530824 through http://www.medscape.com.

Bartlett, J. (2005). Methicillin-Resistant Staphylococcus aureus and Clostridium difficile - Deal Bacterial Threats. Medscape [On-line information]. Available online at http://www.medscape.com/viewarticle/520186 through http://www.medscape.com.

(2006). Community-Acquired MRSA a Leading Cause of Skin and Soft-Tissue Infections. Medscape from Reuters Health Information [On-line information]. Available online at http://www.medscape.com/viewarticle/524998 through http://www.medscape.com.

David, M. et. al. (2006 April 21). Contrasting Pediatric and Adult Methicillin-resistant Staphylococcus aureus Isolates. Medscape from Emerg Infect Dis 12(4): 631-637. Available online at http://www.medscape.com/viewarticle/529448 through http://www.medscape.com.