Treatment of Suspected Staphylococcus aureus Infections When Methicillin Resistance Status is Unknown
For suspected S. aureus infections when methicillin resistance status is unknown, empiric therapy should include coverage for MRSA only in patients with risk factors for antimicrobial resistance, in settings where >10-20% of S. aureus isolates are methicillin-resistant, or in units where MRSA prevalence is unknown. 1
Assessing Risk for MRSA
Before selecting empiric therapy, evaluate for MRSA risk factors:
- Prior intravenous antibiotic use within 90 days 1
- Treatment in a unit where >10-20% of S. aureus isolates are methicillin-resistant 1
- Treatment in a unit where MRSA prevalence is unknown 1
- Prior detection of MRSA by culture or screening 1
- Septic shock or severe illness 1
- Five or more days of hospitalization prior to infection 1
Empiric Treatment Algorithm
1. If MRSA Coverage Indicated (Risk Factors Present):
First-line options:
- Vancomycin (IV): 30-60 mg/kg/day divided in 2-4 doses 2
- Target trough levels: 15-20 mg/mL for serious infections 2
- Linezolid (IV/PO): 600 mg twice daily 2, 3
- Particularly effective for pneumonia and skin infections 2
Alternative options:
- Daptomycin (IV): 6 mg/kg once daily 4
- Teicoplanin (IV): 6-12 mg/kg/dose q12h for three loading doses, then daily 2
2. If MRSA Coverage Not Indicated (No Risk Factors):
First-line options for MSSA:
- Oxacillin, nafcillin, or cefazolin (IV) 1, 2
- These are preferred agents for confirmed MSSA infections 1
- For empiric treatment: piperacillin-tazobactam, cefepime, levofloxacin, imipenem, or meropenem 1
Alternative options (e.g., for penicillin allergy):
- Clindamycin (if local resistance <10%) 2
- TMP-SMX for uncomplicated cases 2
- Doxycycline or minocycline for mild infections 2
Treatment by Infection Type
Skin and Soft Tissue Infections:
- Purulent infections: Incision and drainage is primary therapy; add antibiotics for moderate/severe infections 1
- Non-purulent infections: Empiric therapy based on severity and MRSA risk 1
- Duration: 5-10 days for uncomplicated, 7-14 days for complicated infections 2
Bacteremia/Endocarditis:
- MRSA: Vancomycin or daptomycin 2, 4
- MSSA: Antistaphylococcal penicillins (oxacillin, nafcillin) or cefazolin 2, 7
- Duration: Minimum 14 days for uncomplicated, 4-6 weeks for complicated cases 2
Pneumonia:
De-escalation of Therapy
Once culture and susceptibility results are available:
If MSSA confirmed: Switch to oxacillin, nafcillin, or cefazolin 1, 7
- These agents are superior to vancomycin for MSSA infections 7
- More rapid clearance of bacteremia
- Better clinical outcomes
If MRSA confirmed: Continue or adjust MRSA-targeted therapy based on susceptibilities and infection site 2
Important Considerations
- Local resistance patterns: Treatment should be guided by institutional antibiograms 1
- Vancomycin limitations: Increasing MICs, poor tissue penetration, and nephrotoxicity concerns 5, 6
- Source control: Surgical drainage of abscesses and removal of infected devices are essential 2
- Monitoring: Follow clinical response, inflammatory markers, and repeat cultures as needed
Prevention of Recurrent Infections
- Emphasize hygiene measures: cover draining wounds, regular handwashing, avoid sharing personal items 2
- Consider decolonization for recurrent infections: chlorhexidine body washes for 5-14 days 2
By following this approach, you can provide appropriate empiric coverage for S. aureus while avoiding unnecessary broad-spectrum therapy, thereby reducing the risk of antimicrobial resistance and adverse effects.