How do you diagnose the cause of a skin infection, such as MRSA (Methicillin-resistant Staphylococcus aureus) or Staph (Staphylococcus aureus)?

Diagnosing the Cause of Skin Infections (MRSA vs. Staph)

The most definitive method to diagnose the cause of a skin infection is to obtain a culture of the wound, abscess, or lesion, which should be performed for all purulent skin infections to identify the causative organism and determine antibiotic susceptibility patterns. 1

Clinical Assessment and Specimen Collection

• Obtain cultures from purulent material through needle aspiration, tissue biopsy, or wound swabs before starting antibiotic therapy 1
• For abscesses, incision and drainage should be performed, with culture of the purulent material 1
• Blood cultures are recommended for patients with systemic signs of infection (fever, tachycardia, hypotension) 1
• Gram stain of the specimen can provide rapid preliminary identification of the causative organism 1

Laboratory Diagnosis

• Culture and susceptibility testing are essential to differentiate between methicillin-susceptible S. aureus (MSSA) and methicillin-resistant S. aureus (MRSA) 1
• For S. aureus screening, primary culture sites should include the nares and any open skin lesions 1
• For Group A Streptococcus, swabbing of tonsils and posterior pharynx for rapid antigen detection and culture is recommended 1

Clinical Features Suggesting Specific Pathogens

Features Suggesting Staphylococcal Infection:

• Purulent lesions (furuncles, carbuncles, abscesses) 1
• Folliculitis 2
• Recurrent skin infections, especially in young adults 3

Features Suggesting MRSA Specifically:

• Previous history of MRSA infection or colonization within the past year 1
• Failed treatment with beta-lactam antibiotics 4
• Purulent skin infection in areas with high MRSA prevalence 1
• Recurrent abscesses despite appropriate treatment 3

Features Suggesting Streptococcal Infection:

• Erysipelas (fiery red, tender plaque with well-demarcated edges) 1
• Diffuse cellulitis without purulence 1
• Rapidly spreading infection 1

Warning Signs of Severe Infection

• Systemic toxicity (fever >38°C, tachycardia >90 beats/min, hypotension) 1
• Pain disproportionate to physical findings 1
• Violaceous bullae, cutaneous hemorrhage, or skin sloughing 1
• Skin anesthesia 1
• Rapid progression of infection 1
• Gas in the tissue 1
• Elevated laboratory markers (creatinine, C-reactive protein >13 mg/L, creatine phosphokinase 2-3 times upper limit) 1

Special Considerations

• In diabetic foot infections, P. aeruginosa may be a colonizer rather than a pathogen, requiring clinical correlation with culture results 1
• For recurrent skin abscesses, search for local causes such as pilonidal cysts or hidradenitis suppurativa 1
• In immunocompromised patients, broader culture workup may be necessary to identify unusual pathogens 1

Common Pitfalls to Avoid

• Failing to obtain cultures before starting antibiotics can make pathogen identification difficult 1
• Aspiration of uninvolved skin is unhelpful in 75-80% of cellulitis cases 1
• Treating colonization rather than infection (particularly with P. aeruginosa in diabetic foot infections) 1
• Not considering MRSA in areas with high prevalence 1, 4
• Ignoring clinical response when interpreting culture results - some patients improve despite therapy that appears ineffective against cultured organisms 1

By following this diagnostic approach, clinicians can accurately identify the causative organism of skin infections, distinguish between MRSA and other pathogens, and select appropriate antimicrobial therapy based on culture and susceptibility results.