When is MRSA (Methicillin-resistant Staphylococcus aureus) coverage indicated?

When is MRSA Coverage Indicated?

Empirical MRSA coverage should be initiated when patients have prior MRSA infection/colonization within the past year, when local MRSA prevalence exceeds specific thresholds (≥30% for moderate infections, ≥50% for mild infections), or when infection severity makes delayed treatment unacceptably risky. 1

Risk-Based Algorithm for MRSA Coverage

High-Risk Indicators (Strongly Consider MRSA Coverage)

Prior MRSA history is the single most reliable predictor and warrants empirical coverage:

• Previous MRSA infection or colonization within the past 12 months 1
• Up to 29% of patients with prior MRSA develop subsequent infections within 18 months, with 80% occurring at new anatomic sites 2

Patient-Specific Risk Factors (Score-Based Approach)

Assign points to calculate MRSA risk 3:

• 2 points: History of MRSA infection or colonization
• 1 point each: Recent hospitalization, recent antibiotic use, chronic kidney disease, intravenous drug use, HIV/AIDS, diabetes with obesity 1, 3

A risk score of ≥1 has >90% negative predictive value when baseline MRSA prevalence is ≤30%, meaning a score of 0 effectively rules out need for empirical coverage in low-acuity settings 3

Infection Severity Considerations

Severe infections require empirical MRSA coverage regardless of risk factors when delayed treatment poses unacceptable mortality risk 1:

• Systemic signs of infection (fever, tachycardia, hypotension) 1, 4
• Rapidly progressive cellulitis with associated purulent drainage 1, 4
• Severe or extensive disease involving multiple anatomic sites 4, 5
• Penetrating trauma with purulent drainage 1
• Evidence of MRSA infection elsewhere in the body 1

Local Epidemiology Thresholds

Use institutional antibiograms to guide empirical decisions 1:

• Mild soft tissue infections: Cover MRSA when local prevalence ≥50% of S. aureus isolates 1
• Moderate soft tissue infections: Cover MRSA when local prevalence ≥30% of S. aureus isolates 1
• Severe infections or diabetic foot infections: Lower threshold for coverage given consequences of treatment failure 1

Special Clinical Scenarios

Diabetic foot infections have unique considerations 1:

• Long duration of foot wound increases MRSA risk 1
• Presence of osteomyelitis increases MRSA likelihood 1
• Obtain bone specimen when MRSA is suspected in bone infections 1

Nosocomial/postoperative infections require broader coverage 1:

• MRSA has emerged as the leading cause of postoperative infections, especially in complicated skin and soft tissue infections 1
• High suspicion warrants vancomycin or alternative anti-MRSA therapy 1

Critical Pitfalls to Avoid

Do not rely solely on surveillance percentages—the MRSA risk score is significantly more predictive than surveillance data alone (aROC 0.748 vs 0.646, P=0.016) 3

Avoid clindamycin if local resistance rates are ≥10% or if resistance patterns are unknown 4, 5

Never use beta-lactam antibiotics alone (penicillins, cephalosporins) for suspected MRSA—they are completely ineffective 5

Incision and drainage remains the cornerstone for abscesses; antibiotics are adjunctive therapy, not primary treatment 4, 5

When MRSA Coverage is NOT Indicated

Withhold empirical MRSA coverage when:

• Risk score is 0 AND local MRSA prevalence is <30% AND infection is mild 3
• Typical cellulitis without purulent drainage, systemic signs, or risk factors—streptococcal coverage alone is appropriate 1
• Mild infections in immunocompetent patients without healthcare exposure 1, 4