At a glance

Also known as

MRSA

Why get tested?

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

When to get tested?

When your doctor wants to determine if you are colonised with MRSA, often on admission to a health care unit (eg hospital, renal dialysis unit); following treatment of a MRSA infection.

Sample required?

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

Test preparation needed?

None

What is being tested?

These tests detect the presence of methicillin resistant Staphylococcus aureus (MRSA). MRSA are strains of Staphylococcus aureus, or "golden staph", bacteria that are resistant to the antibiotic methicillin and to related "beta lactam" antibiotics such as flucloxacillin and cefalexin. First appearing in the early 1960s, MRSA cases and outbreaks were a problem in confined populations such as hospitals, prisons, and nursing homes for decades.  There is now known to be spread within the community with otherwise healthy people incurring infections from MRSA.

MRSA strains have caused a significant number of severe skin, lung, bone, and heart-related infections that have proven difficult to treat and, in some cases these prove 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 Staphylococcus aureus is a common bacterium that colonises the skin and is found in the nose of about 25-30% of the population.  An Australian study of Staphylococcus aureus isolates collected from sites of infection have shown that MRSA causes over 30% of golden staph infections.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 often shows 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 is sometimes found 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.

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 colonised 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 determine whether a person needs to be isolated in hospital, or 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 culture. This test confirms the presence of MRSA and allows the organisms to be further characterised, however culture often takes 1 to 2 days to yield a result. A swab is collected from the person’s nares (nostrils) and, put onto a special nutrient medium, then incubated and examined for the growth of characteristic MRSA colonies. A swab may be collected from a wound site or skin lesion of a person 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 colonised with these resistant bacteria (a carrier) or all new admissions to the hospital. When an outbreak of MRSA is under investigation, screening of other people (including health care workers, family members and close contacts) may be performed to identify the potential source of 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 colonisation 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, dicloxacillin and cefalexin 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.

When is it requested?

A MRSA screening test may be ordered when a doctor, hospital, community health department, or researcher wants to evaluate potential MRSA colonisation in an individual, their family members, and/or a group of people in the community as the source of a MRSA infection. A person who has been treated for MRSA infection or for MRSA colonisation will often be screened several times to determine whether MRSA is still present or whether the person has "cleared" their carriage.

What does the test result mean?

If either a culture or a molecular test is positive for MRSA, then the person is colonised 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.

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.

Common Questions

Can I get MRSA more than once?

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

Can I be colonised with MRSA and not know it?

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

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 decolonise a patient with MRSA. By eliminating the nasal colonisation, we can protect the patient from infecting their own skin following, for example, after cardiac surgery, and prevent spreading it to others.


Last Review Date: June 8, 2012