How will you tell if it's MRSA (Methicillin-resistant Staphylococcus aureus)?

How to Diagnose MRSA Infections

The definitive diagnosis of MRSA requires obtaining cultures from the infection site before starting antibiotics, as recommended by the Infectious Diseases Society of America. 1

Diagnostic Approach for MRSA

Culture and Gram Stain

• Obtain cultures before starting antibiotics in suspected MRSA infections 1
• Gram stain and culture of pus from carbuncles and abscesses are recommended, though treatment without these studies is reasonable in typical cases 2
• For purulent infections, collect samples through:
  • Needle aspiration of abscess fluid
  • Punch biopsy specimens
  • Wound cultures from draining lesions 1
• Blood cultures should be obtained if systemic symptoms are present 1

Risk Factors to Consider

• Previous MRSA infection or colonization within the past year (strongest predictor) 1
• Prior long-term or inappropriate antibiotic use 1
• Previous hospitalization or contact with healthcare settings 1
• High local prevalence of MRSA (>30% for moderate infections, >50% for mild infections) 1
• Living in crowded conditions 1
• Rapidly progressing infections 1
• Recurrent skin infections despite appropriate wound care 1

Clinical Features Suggestive of MRSA

• Small, irregularly shaped, or indistinct abscesses with ill-defined edges (patients with these characteristics are seven times more likely to have MRSA) 3
• Treatment failure after initial incision and drainage (MRSA-positive patients are more likely to fail treatment within 7 days) 4
• Severe or rapidly progressing infections 1

Diagnostic Algorithm

1. Assess risk factors for MRSA infection
2. Obtain appropriate samples:
   • For purulent lesions: Collect pus via needle aspiration or during incision and drainage
   • For non-purulent lesions: Consider punch biopsy
   • For systemic symptoms: Obtain blood cultures
3. Request both Gram stain and culture with sensitivity testing 2
4. Initiate empiric therapy based on local resistance patterns while awaiting culture results
5. Reevaluate within 24-48 hours if patient was sent home on empiric therapy 1

Common Pitfalls to Avoid

• Failing to obtain cultures before starting antibiotics can lead to inadequate treatment 1
• Not considering MRSA in patients with risk factors, even with smaller abscesses 3
• Overlooking clindamycin resistance (present in up to 50% of MRSA strains) can result in treatment failure 1
• Not reevaluating patients within 24-48 hours when using empiric therapy can delay appropriate treatment 1
• Assuming abscess size predicts MRSA status (studies show that abscess size does not reliably predict which patients will have MRSA) 4

Special Considerations

• For recurrent MRSA infections, consider screening for colonization and implementing decolonization strategies 1
• In areas with high MRSA prevalence, empiric coverage for MRSA may be warranted even before culture results 1
• The prevalence of MRSA in skin abscesses has increased significantly over time, now accounting for over 50% of all abscesses in many settings 5

Remember that while clinical features can raise suspicion for MRSA, definitive diagnosis requires laboratory confirmation through culture and susceptibility testing.