Trimethoprim-Sulfamethoxazole Dosing for MRSA in a 3-Year-Old Weighing 19.6 kg
For this 3-year-old patient weighing 19.6 kg with MRSA infection, administer trimethoprim-sulfamethoxazole 8-12 mg/kg/day based on the trimethoprim component, divided into 2 doses every 12 hours, which translates to approximately 157-235 mg of trimethoprim per day (or roughly 1 to 1.5 pediatric tablets twice daily), for a duration of 7-10 days depending on infection type and clinical response. 1
Precise Dose Calculation
For this specific patient:
- Weight-based dosing: Using 10 mg/kg/day of trimethoprim (middle of the 8-12 mg/kg range), this child requires 196 mg trimethoprim daily 1
- Practical dosing: This equals approximately 98 mg trimethoprim per dose, given twice daily 1
- Tablet formulation: Using pediatric single-strength tablets (40 mg trimethoprim/200 mg sulfamethoxazole), give 2.5 tablets twice daily, or round to 2 tablets twice daily for practical administration 2
- Alternative using suspension: If liquid formulation is available and preferred, calculate based on 40 mg trimethoprim/200 mg sulfamethoxazole per 5 mL 2
Treatment Duration by Infection Type
The duration varies significantly based on the specific MRSA infection:
- Skin and soft tissue infections (uncomplicated): 7 days is typically sufficient 1
- Complicated skin infections or abscess with cellulitis: 7-14 days depending on clinical response 1
- Osteomyelitis: Greater than 6 weeks, though TMP-SMX is not first-line for this indication 1
- Bacteremia (uncomplicated): 2 weeks minimum 1
- Bacteremia (complicated): 4-6 weeks 1
Critical Clinical Considerations
TMP-SMX is bactericidal but has limited published efficacy data for MRSA compared to vancomycin or linezolid, making it a second-line oral option rather than first-line therapy. 1
When TMP-SMX is Appropriate:
- Mild to moderate skin and soft tissue infections after incision and drainage 1, 3
- Outpatient management of uncomplicated MRSA infections 3
- When clindamycin resistance exceeds 10% locally 1
- As step-down oral therapy after initial IV treatment in stable patients 1
When TMP-SMX Should NOT Be Used:
- Infective endocarditis or suspected endovascular infection - use vancomycin or daptomycin instead 1
- MRSA pneumonia - vancomycin, linezolid, or clindamycin are preferred 1
- Severe systemic infection requiring hospitalization - initiate with IV vancomycin 1
- Children under 2 months of age - TMP-SMX is contraindicated 2
Monitoring and Follow-Up
- Reassess at 48-72 hours: If no clinical improvement, consider inadequate source control, deeper infection, or alternative pathogen 4
- Watch for adverse effects: TMP-SMX can cause hepatotoxicity (rare but serious in children), rash, and gastrointestinal symptoms 5
- Obtain cultures before starting therapy: Confirm MRSA susceptibility to TMP-SMX, as resistance rates vary geographically 6
Important Caveats
The evidence supporting TMP-SMX for MRSA is weaker than for other agents. While guidelines list it as an option, the 2005 IDSA guidelines note "bactericidal; efficacy poorly documented" and "limited published efficacy data" 1. The most robust pediatric data supports its use for uncomplicated skin abscesses after drainage, where one high-quality trial showed 7% absolute improvement in cure rates compared to placebo 3.
For serious MRSA infections requiring hospitalization, vancomycin 40 mg/kg/day IV in 4 divided doses remains the parenteral drug of choice in children. 1 Linezolid 10 mg/kg/dose every 8 hours (not exceeding 600 mg/dose) is the preferred oral alternative when TMP-SMX is inadequate or contraindicated 1.
Clindamycin 30-40 mg/kg/day divided into 3-4 doses is generally preferred over TMP-SMX for oral MRSA therapy in children when local resistance is <10%, as it has superior clinical data and covers both MRSA and streptococcal species. 1, 4