When should a patient with symptoms of a skin or soft tissue infection, particularly those with a history of recent hospitalization, contact with someone with Methicillin-resistant Staphylococcus aureus (MRSA), or a weakened immune system, be tested for MRSA?

Testing for MRSA in Skin and Soft Tissue Infections

Obtain bacterial culture with susceptibility testing for any patient with purulent drainage, systemic signs of infection, or risk factors for MRSA before initiating antibiotic therapy. 1, 2

When to Obtain Cultures

Mandatory Culture Situations

• All purulent lesions (abscesses, furuncles, carbuncles) require culture of drainage material before or at the time of incision and drainage 1, 2

• Systemic toxicity indicators warrant immediate blood cultures and wound cultures: fever >38°C or hypothermia, tachycardia (>100 beats/min), hypotension (systolic BP <90 mm Hg), or abnormal white blood cell count 1

• Severe infections with signs of deeper involvement—including bullae, skin sloughing, cutaneous hemorrhage, skin anesthesia, rapid progression, or gas in tissue—require aggressive diagnostic workup with Gram stain and culture from needle aspiration or punch biopsy 1

• Hospitalized patients or those with malignancy, severe systemic features, or unusual predisposing factors should have blood cultures obtained 2

Specimen Collection Technique

• Aspirate purulent material using a sterile needle and syringe from the abscess cavity; if insufficient, obtain tissue biopsy or curettage from the wound base after cleansing and debridement 3

• Avoid swab specimens as they provide less accurate results and may miss the causative organism 3

• Send specimens promptly in sterile containers for both aerobic culture and Gram stain 3

Risk Factors That Should Prompt MRSA Testing

Patient History Factors

• Previous MRSA infection or colonization within the past year is the most reliable predictor of current MRSA infection 1, 2

• Recent hospitalization or residence in nursing homes increases MRSA risk 1

• Recent antibiotic use, particularly quinolones or macrolides, elevates MRSA probability 1

• Contact with someone with known MRSA infection or colonization 2

Clinical Presentation Factors

• Penetrating trauma, especially from illicit drug use, should raise suspicion for MRSA 2

• Evidence of MRSA infection elsewhere on the body or known nasal colonization with MRSA 2

• Infection not responding to beta-lactam antibiotics after 24-48 hours suggests resistant organisms including MRSA 1

High-Risk Populations

• Immunocompromised patients including those with diabetes mellitus, chronic wounds, or weakened immune systems 1

• Specific groups at increased risk: children <2 years, athletes (contact sports), injection drug users, military personnel, inmates, and homosexual males 1

Role of MRSA Nasal Screening

• MRSA nasal swab PCR has a positive predictive value of 70-85.7% and specificity of 92-93.9% for predicting MRSA wound infection 4, 5

• Positive nasal screen increases the probability of MRSA SSTI substantially (positive likelihood ratio 7.4-10.39), making it more accurate than clinical risk factors alone 4, 5

• Negative nasal screen reduces but does not eliminate the probability of MRSA SSTI (negative predictive value 72.8-92.0%) 4, 5

• Consider nasal screening when rapid results would influence empiric antibiotic selection, particularly in settings where MRSA prevalence is uncertain 4, 5

Local Epidemiology Considerations

• High local MRSA prevalence (>30% of S. aureus isolates for severe infections, >50% for mild infections) should prompt empiric MRSA coverage and culture confirmation 1

• Geographic variation is substantial: North America reports 35.9% MRSA rates, Latin America 29.4%, and Europe 22.8% among S. aureus isolates 1

Common Pitfalls to Avoid

• Failing to culture purulent lesions before initiating antibiotics leads to inability to guide definitive therapy 2

• Using swab specimens instead of aspirated material or tissue biopsy reduces diagnostic accuracy 3

• Treating with beta-lactams alone when MRSA is suspected or local prevalence is high results in treatment failure 2

• Not performing incision and drainage for abscesses—drainage is the primary treatment and antibiotics are adjunctive 1, 2

• Overlooking the need for reevaluation at 24-48 hours when empiric therapy is initiated without culture data 1

Clinical Algorithm for Testing Decisions

Step 1: Assess for purulent drainage or collection → If present, obtain culture before or during drainage 1, 2

Step 2: Check for systemic signs (fever, tachycardia, hypotension, abnormal WBC) → If present, obtain blood cultures and wound cultures 1

Step 3: Evaluate risk factors (prior MRSA, recent hospitalization, recent antibiotics, immunocompromised) → If ≥2 risk factors present, obtain culture and consider nasal screening 1, 2, 4

Step 4: Consider local MRSA prevalence → If >30% for severe or >50% for mild infections, obtain culture to guide therapy 1

Step 5: For patients without purulent drainage but with cellulitis and risk factors, consider needle aspiration or punch biopsy for culture 1