MRSA Testing and Diagnosis
If your doctor suspects you have methicillin-resistant Staphylococcus Aureus or MRSA, he will order specific tests to make an accurate diagnosis. The type of test he orders will be dependent upon a number of things. Your doctor will consider your symptoms, and the site of possible MRSA infection when he orders diagnostic tests. A MRSA screen will look for the presence of MRSA and no other pathogens.
Before we explore the various types of MRSA testing it is important you understand how a culture is performed. A microbiological culture is a method in which specific microbial organisms are allowed to multiply in a culture media in the lab under controlled conditions. For example, a sample of the drainage from a wound can be placed in a petri dish that contains a thin layer of an agar-based growth medium. Agar is a gelatinous substance that is derived from seaweed. The petri dish is then incubated at the best temperature to promote growth dependent upon the source of the sample. Cultures from humans are best grown at 37 degrees Celsius (98.6 F). A culture can take approximately three to five days to grow completely.
Staphylococci (staph) bacteria cells will be round and they will appear in clusters when looking at them under a microscope. They will resemble clusters of grapes. There are various types of staph bacteria, but one of the ways staph aureus is identified is it forms a large yellow colony on the dish. If a sample is determined to be MRSA, the lab can send a ID and Sensitivity report to the physician. This report will say the sample sent to the lab is MRSA, and it will list the antibiotics the bacteria is the most susceptible to.
Obtaining Samples to be Tested for MRSA
There are various places on and in the body where MRSA can grow. As mentioned before, your doctor will determine where to get a culture to test for MRSA. Let’s start with the most common place for MRSA, the skin.
If there is a skin infection present, a small biopsy of the skin or a sample of the drainage from the infection will be obtained and sent to the lab for testing. If your doctor suspects MRSA, he may begin antibiotic treatment even before he gets the results back from the culture. This can keep the infection from progressing even farther.
If an individual has pneumonia that is not getting better despite treatment a sputum sample will be obtained. This sample can be obtained by having the individual expectorate into a cup. A respiratory therapist can obtain a sample with a respiratory lavage or a pulmonologist may insert a scope into the lungs to get a sample of the secretions.
If the doctor suspects MRSA is in the bloodstream, he may order blood cultures. This is done through a simple blood draw.
Since MRSA can also invade the kidneys, the doctor may send a sample of urine to the lab to be tested.
Individuals can be colonized with MRSA but not have symptoms. This colonization can allow them to spread MRSA to others. MRSA if often carried in the nose and on the skin. A nasal culture is obtained by inserting specially prepared cotton swabs approximately 1 cm into the nare. The swabs are then rotated 2-4 times. The same swabs are then inserted into the other nare and rotated. The swabs are then placed into the transport medium and sealed to be sent to the lab.
Many healthcare facilities require nasal cultures to test patients who are re-admitted to the hospital within 30 days. Other requirements for testing may include patients who are admitted from a long-term health care facility, patients admitted to critical care, dialysis patients and those going for elective surgeries.
There are new tests available that provide rapid screening for MRSA. These new tests can detect the presence of MRSA in just a few hours compared to a few days. It is hard to say how many healthcare facilities are using these tests, and if they will become the standard for MRSA screenings in the future.
HA-MRSA and CA-MRSA
MRSA used to be only a hospital or healthcare facility acquired infection. However, recently there has been a surge in community-acquired MRSA infections. A doctor can determine if the has HA-MRSA or CA-MRSA. The criteria for CA-MRSA is as follows:
- A diagnosis was made in an outpatient setting or by a positive culture within forty-eight hours of admission to the hospital.
- There is no medical history of a MRSA infection.
- The patient has no history of a hospitalization in the past year.
- The patient has no history of being in a nursing home, skilled nursing facility or hospice in the past year.
- The patient has no history of dialysis.
- The patient has no history of recent surgery.
- The patient does not have indwelling catheters or implanted medical devices inside the body.
It can be important for the doctor to know what type of MRSA the patient has. Recent outbreaks of MRSA in the community have been associated with strains that have unique microbiologic properties compared with the traditional hospital-acquired MRSA. These strains suggest they contain properties that allow for MRSA to spread more easily and cause more skin disease. Research is being done right now to compare the biological strains of HA-MRSA and CA-MRSA.
Regardless of the type of MRSA an individual has, it is critical he or she seeks treatment. MRSA that is left untreated can progress into necrotizing fasciitis. This is a rapidly spreading infection of the connective tissue that results in death of the tissue. Necrotizing fasciitis is fatal in about 75% of all cases. There is new evidence to suggest that some strains of CA-MRSA are developing into this fatal skin disease. Therefore, any skin infection that you have should never be ignored. Early intervention could save your life.