When you have no signs or symptoms of an active infection, but a healthcare practitioner wants to determine if you are a MRSA carrier, such as when you are hospitalized
Methicillin-resistant Staphylococcus aureus (MRSA) are strains of Staphylococcus aureus, or "staph," bacteria that are resistant to the antibiotic methicillin as well as to related beta-lactam antibiotics, such as oxacillin, penicillin, amoxicillin, and cephalosporins, that are used to treat ordinary staph infections. MRSA testing detects the presence of MRSA in a patient's sample.
Staphylococcus aureus frequently colonizes the human skin and is present in the nose of about a third of U.S. adults. It does not usually cause illness or symptoms. However, if there is a break in someone's skin from a wound or surgery, or if someone's immune system is weakened, then colonizing S. aureus can cause an infection.
Widespread use of antibiotics over the past several decades have led to the development of antibiotic-resistant strains of S. aureus. Infection with these strains can cause skin infections and potentially more severe, life-threatening infections such as pneumonia, bloodstream infections (septicemia), or infections at a surgical site. However, according to the Centers for Disease Control and Prevention (CDC), less than 2% of the population carries the type of staph bacteria known as MRSA. [For more on this, see the article Staph Infections and Methicillin-Resistant Staphylococcus aureus.]
MRSA can be spread in healthcare settings and in the community. Contaminated hands, medical equipment, and surfaces in places such as hospitals, clinics, or nursing homes allow the spread of MRSA from colonized or infected patients. In the community, anything that allows for skin-to-skin contact can spread MRSA. This includes sharing personal care items, participating in contact sports, or living or working in close contact with others.
Most hospitals have instituted measures to attempt to eradicate MRSA and to control the spread of MRSA from person to person. Those who test positive for MRSA may be isolated to prevent the spread to others and/or decolonized with a nasal antibiotic ointment and daily cleansing with special antimicrobial wipes. However, exactly how to implement MRSA screening and control in healthcare settings remains somewhat controversial. For example, a 2014 study comparing prevention strategies for bloodstream infections found that treating to eliminate MRSA from intensive care unit (ICU) patients who screen positive was not as effective at reducing these infections as treating all patients admitted to the ICU. This treatment may involve administering antibiotics within the nose and cleansing patients with special antimicrobial wipes. As a result, hospital policies may change in the future and less MRSA screening may be performed in these settings.
How is the sample collected for testing?
A nasal swab is collected by rotating a swab inside each nostril. Occasionally, a swab of a wound infection site or skin lesion is collected.
How is it used?
A methicillin-resistant Staphylococcus aureus (MRSA) screen tests solely for the presence of MRSA and no other microbes. It is primarily used to identify the presence of MRSA in a colonized person so they can be treated to eliminate MRSA and prevent its spread to others.
Often, the test is used to screen for MRSA in patients in hospitals, especially in intensive care units. On a community level, MRSA screening may be used to help determine the source of an outbreak. On a national level, additional testing may inform clinicians and researchers about the unique genetic characteristics of the strains of MRSA circulating in the community or healthcare setting.
MRSA screening tests include:
- Bacterial culture - 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. A screening culture identifies the absence or presence of MRSA and usually takes 1 to 2 days for a result.
- Molecular tests for MRSA screening can detect nasal or wound carriage within hours, allowing for prompt treatment as necessary. The same specimen types are used in a molecular test, but the specimen is analyzed for the genetic markers to identify S. aureus and the mecA gene that confers resistance to methicillin, oxacillin, nafcillin, dicloxacillin, and other similar antibiotics. Molecular MRSA screening is becoming more widespread.
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 healthcare 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.
When is it ordered?
A MRSA screening test may be ordered when a healthcare practitioner, hospital, or health department needs 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 sports team, residents of a nursing home, or healthcare workers, may be tested for MRSA carrier status when an outbreak of MRSA is suspected.
Occasionally, a person who has been treated for MRSA may be screened to determine whether MRSA are still present.
What does the test result mean?
If either a culture or a molecular test is positive for MRSA, then the bacteria are present at the site that was tested, such as the nostrils or a wound site.
If the nasal screen or wound site culture is negative, then MRSA are either not present or are present in numbers too low to be detected by the test.
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.
Can I get MRSA more than once?
Can I be colonized with MRSA and not know it?
How can I protect myself from getting a MRSA infection?
On The Site
Tests: Wound Culture