What is the recommended dosing of Bactrim (trimethoprim/sulfamethoxazole) for a 10-year-old patient weighing 77kg with a facial Staphylococcus (Staph) infection?

Bactrim Dosing for Facial Staph Infection in a 10-Year-Old (77kg)

Recommended Dose

For this 10-year-old patient weighing 77kg with a facial staph infection, prescribe Bactrim at 8-12 mg/kg/day based on the trimethoprim component, divided into 2 doses every 12 hours, which translates to 616-924 mg trimethoprim per day (or 308-462 mg trimethoprim twice daily), equivalent to approximately 2-3 double-strength tablets twice daily for 7-10 days. 1, 2

Weight-Based Calculation Details

• This patient's weight of 77kg significantly exceeds typical pediatric weights, placing them in the upper range of pediatric dosing or potentially requiring adult-equivalent dosing 1
• Using the standard pediatric formula of 8-12 mg/kg/day of trimethoprim component:
  • Lower range: 77kg × 8 mg/kg = 616 mg trimethoprim/day (308 mg twice daily)
  • Upper range: 77kg × 12 mg/kg = 924 mg trimethoprim/day (462 mg twice daily) 1, 2
• Since one double-strength tablet contains 160 mg trimethoprim, this translates to approximately 2 double-strength tablets twice daily (320 mg trimethoprim per dose) as a practical starting point 3

Specific Considerations for Facial Staph Infections

• For purulent skin infections where MRSA is suspected, Bactrim is an appropriate first-line agent with strong efficacy 2, 4
• If the facial infection is non-purulent cellulitis (no abscess or purulent drainage), you must add a beta-lactam antibiotic such as amoxicillin or cephalexin because Bactrim has poor activity against beta-hemolytic streptococci, which are common pathogens in non-purulent cellulitis 2, 4
• Treatment duration should be 7-10 days based on clinical response, with consideration for extending to 10 days if there is slow improvement 1, 2

Critical Safety Considerations

• Verify the patient is at least 2 months old (this 10-year-old clearly meets this requirement) 1, 3
• Screen for contraindications including documented sulfa allergy, megaloblastic anemia due to folate deficiency, and known hypersensitivity to trimethoprim or sulfonamides 1, 4
• Monitor for serious adverse effects including Stevens-Johnson syndrome, toxic epidermal necrolysis, and thrombocytopenia, particularly with prolonged therapy 1
• Ensure adequate hydration to prevent crystalluria 3

Practical Prescribing Approach

• Start with 2 double-strength tablets (800mg/160mg) twice daily for 7-10 days as this provides 320 mg trimethoprim per dose (640 mg/day total), which falls within the recommended 8-12 mg/kg/day range for this patient 2, 3
• If clinical response is inadequate after 48-72 hours, consider increasing to 3 double-strength tablets twice daily (960 mg trimethoprim/day total), though this approaches adult maximum dosing 5
• The higher dose (320mg/1600mg twice daily) has been studied in adults with MRSA skin infections and showed similar efficacy to standard dosing, though it did not demonstrate clear superiority 5

Important Clinical Caveats

• Facial infections warrant careful monitoring due to proximity to critical structures and risk of complications - ensure close follow-up within 48-72 hours 2
• If there is an abscess component, incision and drainage is essential and should be performed in addition to antibiotic therapy 6
• Consider culture and sensitivity testing, particularly for facial infections, to guide therapy if initial treatment fails 2
• For severe infections with systemic symptoms, bacteremia, or lack of response to oral therapy, consider hospitalization and IV therapy at 8-12 mg/kg/day (based on trimethoprim) divided into 4 doses 4, 7