What is the appropriate dosing of Bactrim (trimethoprim/sulfamethoxazole) for a wound positive for Methicillin-resistant Staphylococcus aureus (MRSA)?

Bactrim (TMP-SMX) Dosing for MRSA Wound Infections

For MRSA-positive wound infections, the recommended dosage of trimethoprim-sulfamethoxazole (TMP-SMX) is 1-2 double-strength tablets (160mg/800mg) twice daily for 7-14 days. 1

Dosing Recommendations

Adult Dosing

• Oral therapy: 1-2 double-strength tablets (160mg/800mg) twice daily 1
• Duration: 7-14 days, depending on clinical response 1, 2
• Intravenous option: 8-12 mg/kg/day (based on trimethoprim component) in 2-4 divided doses for severe infections 1

Pediatric Dosing

• Children: Trimethoprim 4-6 mg/kg/dose, sulfamethoxazole 20-30 mg/kg/dose orally every 12 hours 1
• Not recommended for children under 2 months of age 1

Clinical Decision Making Algorithm

1. Assess infection severity:

   • Uncomplicated infection (small abscess, minimal surrounding cellulitis): Incision and drainage may be sufficient 1
   • Complicated infection (any of the following): TMP-SMX indicated when:
     • Multiple sites of infection
     • Extensive or rapidly progressing cellulitis
     • Systemic symptoms (fever, elevated WBC)
     • Immunocompromised patient
     • Extremes of age
     • Difficult-to-drain location (face, hand, genitalia)
     • Failed drainage-only approach 1

2. Treatment approach:

   • Primary intervention: Incision and drainage for purulent collections 1
   • Adjunctive antibiotics: Add TMP-SMX for complicated infections or when drainage is incomplete 1

Evidence Quality and Considerations

The recommendation for TMP-SMX dosing is supported by the 2014 IDSA guidelines for MRSA infections 1. A randomized controlled trial showed that TMP-SMX at doses of 320mg/1600mg twice daily for 7 days resulted in higher cure rates (80.5%) compared to placebo (73.6%) for drained abscesses in areas with high MRSA prevalence 2.

Interestingly, a 2011 study found no significant difference in clinical resolution between standard dose (160mg/800mg twice daily) and high dose (320mg/1600mg twice daily) TMP-SMX for MRSA skin infections 3. This suggests that the standard dosing may be adequate for most uncomplicated cases.

Important Caveats

• Beta-hemolytic streptococcal coverage: TMP-SMX has good activity against MRSA but poor activity against beta-hemolytic streptococci. Consider adding coverage for streptococci if this is a concern 1
• Monitoring: Watch for gastrointestinal side effects, which are usually mild 2
• Resistance concerns: Resistance rates to TMP-SMX among MRSA can vary by region 4
• Alternative options if TMP-SMX is contraindicated or not tolerated:
  • Doxycycline 100mg twice daily
  • Clindamycin 300-450mg three times daily
  • Linezolid 600mg twice daily (more expensive) 1

Special Populations

• Pregnancy: TMP-SMX is pregnancy category C/D and not recommended for women in the third trimester 1
• Renal impairment: Dose adjustment required when creatinine clearance is <30 ml/min 5
• Pediatrics: Not recommended for children under 2 months of age 1

Remember that incision and drainage remains the cornerstone of therapy for purulent collections, with antibiotics serving as adjunctive therapy for complicated cases or when drainage is incomplete.