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 patient, or to detect if these resistant bacteria remain at a wound site after the patient has been treated for a MRSA infection. On a community level, screening may be used to help determine the source of an outbreak and on a national level used to evaluate the genetic characteristics of an identified MRSA strain.
The most widely used test to identify MRSA colonization is culture. This test is definitive but takes time, usually 1 to 2 days. A nasal swab is collected from the nares (nostrils) of an asymptomatic person and cultured - put into a special nutrient broth, or spread onto a nutrient gel, incubated, and then examined for the growth of characteristic MRSA colonies. Occasionally, a swab is collected from the 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 a carrier (everyone or specific target groups). Healthcare workers and the family members of carriers may also be screened for MRSA. When an outbreak of MRSA occurs in the community, numerous MRSA screens may be performed to help identify the source of the infection. 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 certain genetic components of MRSA, such as the mecA gene, in the samples of possible carriers. The mecA gene confers resistance to the antibiotics meticillin, and flucloxacillin. When a staphylococcus is resistant to these antimicrobial agents, other similar antibiotics, such as the cephalosporins or amoxicillin/clavulante, are not effective. While this testing for mecA is not yet widely used, it does 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 the rapid and more expensive molecular test.
One or more MRSA screening tests may be requested 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 contact, such as a rugby 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.
If a screen is positive for MRSA, then the patient is a carrier. 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 very low numbers.
If a molecular test confirms the presence of mecA in S. aureus, then the organism is classified as MRSA.
A sampling of positive MRSA tests may be subjected to further testing to help investigate the spread of MRSA within a community or region but are not often used in the treatment of an individual patient. They include pulsed-field gel electrophoresis (PFGE), which can identify the type and subtype of S. aureus strains, and DNA testing, which can be used to look at the genetic material of the microorganism and to detect the presence or absence of mecA, which confers resistance to methicillin, and flucloxacillin.
In a research setting, DNA testing is also being performed to detect the presence of the Panton-Valentine leukocidin (PVL) gene. This gene is associated with the production of a toxin that can greatly exacerbate the complications associated with MRSA infections and can occasionally prove fatal.
Public awareness of MRSA and efforts to contain its spread are growing. Doctors are being encouraged to request MRSA screening on their patients with skin infections and in any cases where they suspect a MRSA carrier. Standard courses of antibiotics may be adequate to treat regular staph infections but often lead to treatment failure in patients with MRSA.
This article was last reviewed on 5 November 2008. | This article was last modified on 1 November 2011.
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