Diagnosis and Testing of MRSA Infections

A Skin Infection May Very Likely be MRSA

The CDC encourages you to consider MRSA in the differential diagnosis of skin and soft tissue infections (SSTIs) compatible with S. aureus infections, especially those that are purulent (fluctuant or palpable fluid-filled cavity, yellow or white center, central point or “head,” draining pus, or possible to aspirate pus with needle or syringe). A patient’s presenting complaint of “spider bite” should raise suspicion of an S. aureus infection.

Recent data suggest that MRSA in the community is increasing. The spectrum of disease caused by MRSA appears to be similar to that of Staphylococcus aureus in the community. SSTIs, specifically furuncles (abscessed hair follicles or “boils”), carbuncles (coalesced masses of furuncles), and abscesses, are the most frequently reported clinical manifestations. The role of MRSA in cellulitis without abscess or purulent drainage is less clear since cultures are rarely obtained.

 

Obtaining Cultures

In general, a culture should be obtained from the infection site and sent to the microbiology laboratory. If S. aureus is isolated, the organism should be tested as follows to determine which antibiotics will be effective for treating the infection.

Skin Infection Culturing

Obtain either a small biopsy of skin or drainage from the infected site. A culture of a skin lesion is especially useful in recurrent or persistent cases of skin infection, in cases of antibiotic failure, and in cases that present with advanced or aggressive infections.

Pneumonia Culturing

Obtain a sputum culture (expectorated purulent sputum, respiratory lavage, or bronchoscopy).

Bloodstream Infection Culturing

Obtain blood cultures using aseptic techniques.

Urinary Infection Culturing

Obtain urine cultures using aseptic techniques.

Laboratory Testing

• Laboratory Detection of Oxacillin/Methicillin-resistant Staphylococcus aureus
• Use of an Inferred PFGE Algorithm, Emerging Infections Program/Active Bacterial Core (ABCs) Surveillance Invasive MRSA Project
• Molecular Typing of Oxacillin-resistant Staphylococcus aureus [PDF - 24 pages]
• M.A.S.T.E.R. (Multi-level Antimicrobial Susceptibility Testing Resources)

Distinguishing CA-MRSA from HA-MRSA

Based on Patient History

Persons with MRSA infections that meet all of the following criteria likely have CA-MRSA infections:

• Diagnosis of MRSA was made in the outpatient setting or by a culture positive for MRSA within 48 hours after admission to the hospital.
• No medical history of MRSA infection or colonization.
• No medical history in the past year of:
  • Hospitalization
  • Admission to a nursing home, skilled nursing facility, or hospice
  • Dialysis
  • Surgery
• No permanent indwelling catheters or medical devices that pass through the skin into the body.

Biological Differences of CA-MRSA and HA-MRSA

Recently recognized outbreaks of MRSA in community settings have been associated with strains that have some unique microbiologic and genetic properties compared with the traditional hospital-based MRSA strains, suggesting some biologic properties (e.g., virulence factors) may allow the community strains to spread more easily or cause more skin disease. Additional studies are underway to characterize and compare the biologic properties of HA-MRSA and CA-MRSA strains. There are at least three different S. aureus strains in the United States that can cause CA-MRSA infections. CDC continues to work with state and local health departments to gather organisms and epidemiologic data from known cases to determine why certain groups of people get these infections.

Reporting MRSA to State Health Departments

MRSA is reportable in several states. The decision to make a particular disease reportable to public health authorities is made by each state, based on the needs of that individual state. To find out if MRSA is reportable in your state, contact your state health department.