How can you diagnose Methicillin-resistant Staphylococcus aureus (MRSA) of the skin?

Diagnosing MRSA Skin Infections

Purulent drainage from a skin lesion is the most reliable clinical indicator of MRSA skin infection, and all suspected MRSA skin lesions should be cultured to confirm the diagnosis and determine antibiotic susceptibility. 1

Clinical Presentation

MRSA skin infections typically present with the following characteristics:

• Purulent lesions such as abscesses, furuncles (boils), or carbuncles, often with surrounding cellulitis 1
• Lesions that are often misinterpreted as spider or insect bites 2
• Tender, erythematous, fluctuant abscesses that may be solitary or multiple 3
• Common locations include axillary regions, buttocks, and thighs 3
• Lesions that fail to respond to standard beta-lactam antibiotics (penicillins, cephalosporins) 3, 4

Diagnostic Approach

When to Suspect MRSA

You should suspect MRSA skin infection in the following scenarios:

• Presence of purulent drainage or collection 1
• Skin infection associated with penetrating trauma, especially from illicit drug use 1
• Evidence of MRSA infection elsewhere on the body 1
• Known nasal colonization with MRSA 1
• Skin infection that fails to respond to beta-lactam antibiotics 3, 4
• Systemic signs of infection (fever, tachycardia, hypotension) with skin infection 1

Diagnostic Testing

• Bacterial culture is essential for definitive diagnosis of MRSA skin infections 1
• For abscesses and purulent lesions, obtain culture of the drainage material 1, 3
• For non-purulent cellulitis, cultures are generally not necessary unless there are systemic signs of infection or risk factors 1
• Blood cultures should be obtained in patients with malignancy, severe systemic features, or unusual predisposing factors 1

Risk Factors for MRSA

Consider MRSA in patients with the following risk factors:

• History of previous MRSA infection or colonization 1
• Close contact with someone known to have MRSA 2
• Recent antibiotic use 1
• Participation in contact sports or weight lifting 3
• Shared equipment or personal items in athletic settings 3
• Living in crowded conditions (military barracks, prisons, dormitories) 2, 3

Distinguishing Features from Other Skin Infections

MRSA skin infections can be distinguished from other skin infections by:

• More frequent presentation as abscesses rather than diffuse cellulitis 2, 5
• Increased virulence and propensity to cause purulent infections 6
• Failure to respond to first-line beta-lactam antibiotics 1, 4
• Rapid progression and more severe local inflammation 1
• Increased likelihood of recurrence 1

Management Considerations

Once MRSA is diagnosed or strongly suspected:

• Incision and drainage is the primary treatment for abscesses 1, 3
• Appropriate antibiotic therapy should be initiated based on local susceptibility patterns 1, 5
• Options for MRSA treatment include vancomycin (IV), linezolid, daptomycin, or oral therapy with doxycycline, clindamycin, or trimethoprim-sulfamethoxazole 1, 7
• Culture results should guide definitive antibiotic therapy 1, 5
• Consider decolonization for recurrent infections with mupirocin nasal ointment and chlorhexidine body washes 1

Common Pitfalls

• Misdiagnosing MRSA abscesses as spider bites 2
• Failing to culture purulent skin lesions 1, 5
• Treating with beta-lactam antibiotics alone when MRSA is suspected 1, 4
• Not performing incision and drainage for abscesses 1, 3
• Overlooking the need for decolonization in recurrent cases 1

Remember that while MRSA is an important pathogen in skin infections, it is an unusual cause of typical non-purulent cellulitis, where streptococci remain the predominant pathogens 1.