How do you diagnose a breast infection as MRSA?

Diagnosing MRSA in Breast Infections

To diagnose MRSA in a breast infection, obtain culture specimens from deep tissue via aspiration of purulent material or tissue biopsy after cleansing the wound, and send for aerobic culture with antimicrobial susceptibility testing before starting antibiotics. 1

Specimen Collection Technique

The optimal approach for identifying MRSA requires proper specimen collection:

• Aspirate purulent secretions using a sterile needle and syringe from the abscess cavity 1
• Obtain tissue biopsy or curettage from the base of the wound after cleansing and debridement if aspiration is insufficient 1
• Avoid swab specimens, as they provide less accurate results and may miss the causative organism 1
• Send specimens promptly in sterile containers for both aerobic culture and Gram stain 1

Clinical Context for MRSA Suspicion

MRSA should be strongly suspected in breast infections based on emerging epidemiologic data showing it accounts for 19-58% of all breast abscess cultures. 2, 3 Consider MRSA particularly when:

• Prior history of MRSA infection in the patient or close contacts 1
• High local prevalence of MRSA in your community or institution 1
• Clinically severe infection with systemic symptoms 1
• Postpartum/lactating women, where MRSA is the predominant organism (50.8% of S. aureus isolates) 2, 4
• Failure to respond to beta-lactam antibiotics within 24-48 hours 1

Diagnostic Workflow

For infected breast lesions, follow this algorithmic approach:

1. Perform ultrasound examination to confirm abscess presence and guide aspiration 4
2. Cleanse the overlying skin before obtaining specimens 1
3. Aspirate or drain the abscess and collect purulent material 1
4. Request both Gram stain and culture with susceptibility testing 1
5. Specify MRSA detection if your laboratory offers molecular testing (NAAT) for rapid identification 1

Important Caveats

Do not culture clinically uninfected wounds, as this provides no therapeutic benefit and may lead to inappropriate antibiotic use 1. However, all purulent breast infections should be cultured given the high MRSA prevalence in this population 2, 3.

Polymicrobial infections occur in 22-39% of breast abscesses, so culture results may reveal mixed flora including anaerobes (particularly Bacteroides species in 14-28% of cases) 3, 5. This finding doesn't exclude MRSA as a co-pathogen.

Empirical Coverage Considerations

While awaiting culture results, empirical MRSA coverage should be initiated in breast infections given that MRSA represents 23-58% of S. aureus isolates in breast abscesses 2, 3, 5. The IDSA guidelines support empirical MRSA therapy when local prevalence is high or infection is severe 1.

All MRSA isolates from breast abscesses show consistent susceptibility to:

• Trimethoprim-sulfamethoxazole (100% sensitive) 4, 3, 5
• Vancomycin (100% sensitive) 4, 3, 5
• Clindamycin (100% sensitive in most series) 3, 5

Levofloxacin shows poor activity with only 29% susceptibility in breast MRSA isolates 3.