MRSA screening
At a Glance
Why Get Tested?
To detect MRSA carrier status
When to Get Tested?
When your doctor wants to determine if you are carrying an MRSA bacteria and may therefore potentially be at risk of developing complications after surgery or other invasive medical procedures
Sample Required?
Swab of nose and skin swab from axilla (armpit) and groin; occasionally swab of wound infection site or skin lesion
The Test Sample
What is being tested?
This test detects the presence of, and sometimes evaluates the genetic characteristics of, methicillin resistant Staphylococcus aureus (MRSA). MRSA is an antibiotic resistant variety of Staphylococcus aureus (S. aureus) or ‘Staph’. MRSA is resistant to the antibiotic methicillin and to related ‘beta lactam’ antibiotics such as flucloxacillin, dicloxacillin and cephalosporins. It is therefore difficult to treat. MRSA is a significant cause of severe skin, lung, bone and heart-related infections. Historically, MRSA has been associated with hospital infections and rapidly developed resistance to multiple classes of antibiotics. Not surprisingly, hospitals have instituted measures to prevent the spread of MRSA from person to person. Most large hospitals test patients who are admitted for the presence of MRSA and put individuals who carry this microorganism in a single room. Evidence suggests that patient isolation when combined with other infection control measures, can reduce the spread of MRSA.
MRSA has been a challenge for infection control as Staph is a common bacterium that colonises the skin and exists in the nose of about 25-30% of the population. Of the expected 2 billion individuals carrying Staph worldwide, it is estimated that between 2 million and 53 million carry MRSA. In patients with chronic wounds, intravenous catheters or patients treated with antibiotics this common variety of susceptible Staph can often be replaced with MRSA. Such patients are at particular risk of MRSA colonisation and infection. Antibiotic overuse at both the patient and institutional level increases the risk of MRSA colonisation and infection.
Unfortunately, in the last few years the number of MRSA infections has increased and a new variety of MRSA has been recognised. This new variety, called community-acquired MRSA, is spreading outside healthcare facilities and can cause infections in young, previously healthy people. Community-acquired MRSA is frequently genetically distinct from MRSA found in the hospital setting. Community-acquired MRSA also differs from hospital-acquired strains in other significant ways, such as in their ability to cause skin infections and severe pneumonia.
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.
The Test
How is it used?
A MRSA screen is a test that looks solely for the presence of MRSA and detects no other pathogens. Even Staph that is susceptible to antibiotics is not detected by the test.
There are different ways of performing this test. The traditional way to detect MRSA is by culture. This test uses of highly selective culture media and is very accurate. However, it usually takes usually 2 days to get a result. Faster ways of MRSA screening rely on molecular methods. These new methods produce results within 24 hours and are more sensitive than traditional cultures. Molecular methods test for certain genetic components of MRSA, such as the mecA gene which is responsible for resistance to methicillin and other beta lactam antibiotics.
When is it requested?
One or more MRSA screening tests may be ordered prior to admission to a hospital to determine whether the patient is carrying MRSA. Occasionally, a person who has been treated for MRSA infection or for MRSA colonisation may be screened to evaluate the success of the treatment.
What does the test result mean?
If a screening test from at least one swab collected is positive for MRSA, then the patient is a carrier. If a molecular test confirms the presence of mecA in S. aureus, then the organism is classified as MRSA.
Is there anything else I should know?
Some positive MRSA tests may be subjected to further molecular subtyping to help investigate the spread of MRSA within a community or region. However, these tests are not often used in the treatment of individual patients. These tests include pulsed-field gel electrophoresis (PFGE), which can identify the type and subtype of S. aureus strains, and molecular testing, which can be used to look at the genetic material of the microorganism, including virulence markers such as the presence of the Panton-Valentine leukocidin (PVL) gene. This gene is associated with the production of a toxin that can increase the severity of MRSA infections.
Public awareness of MRSA and efforts to contain its spread are growing. Doctors are being encouraged to order MRSA screening for patients with skin infections and in any cases where they suspect a MRSA carrier.
Common Questions
Ask a Laboratory Scientist
Article Sources
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.
