Staph Wound Infections and Methicillin Resistant Staphylococcus aureus

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Laboratory Tests

The purpose of laboratory testing is to identify the bacteria causing the wound infection and to determine the susceptibility of the microorganism to available antibiotics. If an infection is due to MRSA, it should be investigated to determine the source of the infection. This is especially important in community-acquired methicillin resistant S. aureus (CA-MRSA) to prevent further cases from occurring.

Testing for staph infections:

  • The primary test for diagnosis of a staph infection is a culture of the affected area. This may involve a bacterial wound culture using fluid or pus from a wound, a sputum culture, blood culture, a culture of joint fluid (synovial fluid), or even breast milk (in the case of an infected breast). Sometimes, multiple samples are collected to evaluate different body sites or to attempt to detect bacteria that may be present in small numbers.
  • Susceptibility testing is performed if S. aureus are detected in a culture, to determine if the strain that is present is MRSA.

Rapid tests for the detection of MRSA have recently been developed. These tests are molecular-based or use immunoassay to determine if the drug-resistant bacteria are present. Whereas cultures typically take 24-48 hours, these tests provide results in 2-5 hours, allowing for prompt treatment.

Identifying MRSA can sometimes be challenging. The population of staph that a person has tends to be mixed. This means that even if someone has CA-MRSA or healthcare-associated MRSA, not all of the staph present at the infection site will be equally resistant. Since resistant strains may grow more slowly than susceptible strains on the laboratory culture, there is the potential for missing them.

A variety of methods may be used to track different strains of MRSA. These are used in the investigation of the spread of MRSA within a community or region but are not used in the treatment of an individual person.

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