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What tests do medical professionals use to diagnose a MRSA infection?

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. If possible, a sample of pus from a wound, blood, or urine is sent to a microbiology lab and cultured for S. aureus. Deep infections (such as bone) may require removal of a piece of tissue for testing (biopsy). 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. Often there is no material to culture, and doctors treat the person with antibiotics that kill MRSA as well as more common bacteria until more information is available. This is called empiric therapy, meaning that doctors make their best guess on what bacteria are likely to be the cause of infection, until the bacteria have been definitively identified.

Some hospitals may screen patients for carrying MRSA, so that precautions can be taken to avoid spreading MRSA. The same procedure is done by swabbing the skin or inside the nose. These tests help distinguish MRSA infections from other skin changes that often appear initially similar to MRSA, such as spider bites or skin changes that occur with Lyme disease. Many MRSA infections get mistaken for a spider bite. This can cause delayed or incorrect treatment and progression of the MRSA infection.

There are rapid screening tests that can detect the presence of MRSA DNA material (polymerase chain reaction, PCR) in a blood sample in as little as two hours. The test is able to determine whether the genetic material is from MRSA or from less dangerous forms of staph bacteria. It may allow hospitals to start precautions early. It may also allow doctors to quickly tailor the antibiotics to only what is needed; this reduces unnecessary antibiotic use and helps reduce antibiotic resistance. It also may reduce side effects and costs of unnecessary antibiotics. These tests cannot be used alone for the diagnosis of a MRSA infection. They do not provide important details about the antibiotics to which the specific strain is susceptible.

Reviewed on 6/30/2017

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