MRSA Screening

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. Photo source: CDC, Melissa Dankel, James Gathany 

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.

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.

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.

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.