February 7, 2016

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How is a MRSA infection diagnosed?

Most doctors start with a complete history and physical exam of the patient to identify any skin changes that may be due to MRSA, especially if the patient or caretaker mentions a close association with a person who has been diagnosed with MRSA. A skin sample, sample of pus from a wound, or blood, urine, or biopsy material (tissue sample) is sent to a microbiology lab and cultured for S. aureus. If S. aureus is isolated (grown on a Petri plate), the bacteria are then exposed to different antibiotics, including methicillin. S. aureus bacteria that grow well when methicillin is in the culture are termed MRSA, and the patient is diagnosed as MRSA infected. The same procedure is done to determine if someone is a MRSA carrier (screening for a carrier), but sample skin or mucous membrane sites are only swabbed, not biopsied. These tests help distinguish MRSA infections from other skin changes that often appear initially similar to MRSA, such as spider bites and skin changes that occur with Lyme disease. These tests are very important; misidentification of a MRSA infection may cause the patient to be treated with other agents like dapsone (used for spider bites). This can result in progression of the MRSA infection and even other complications due to the dapsone.

In 2008, the U.S. Food and Drug Administration (FDA) approved a rapid blood test (StaphSR assay) that can detect the presence of MRSA genetic material in a blood sample in as little as two hours. The test is also able to determine whether the genetic material is from MRSA or from less dangerous forms of staph bacteria. The test (PCR based) is not recommended for use in monitoring treatment of MRSA infections and should not be used as the only basis for the diagnosis of a MRSA infection. In addition, there are new screening tests that report detecting or ruling out MRSA infections in about five hours. Continue Reading

Reviewed on 2/4/2015