Bactrim Dosing for a 22kg Child with Purulent Cellulitis
For a 22kg child with purulent cellulitis, the recommended dose of trimethoprim-sulfamethoxazole (Bactrim) is trimethoprim 4-6 mg/kg/dose and sulfamethoxazole 20-30 mg/kg/dose given orally every 12 hours.
Dosage Calculation
Based on the child's weight of 22kg:
- Trimethoprim component: 4-6 mg/kg/dose
- Lower range: 22kg × 4 mg/kg = 88 mg per dose
- Upper range: 22kg × 6 mg/kg = 132 mg per dose
- Sulfamethoxazole component: 20-30 mg/kg/dose
- Lower range: 22kg × 20 mg/kg = 440 mg per dose
- Upper range: 22kg × 30 mg/kg = 660 mg per dose
This translates to approximately one regular-strength tablet (80mg TMP/400mg SMX) per dose given twice daily.
Evidence-Based Rationale
The Infectious Diseases Society of America (IDSA) guidelines for the treatment of methicillin-resistant Staphylococcus aureus (MRSA) infections specifically recommend trimethoprim-sulfamethoxazole for purulent cellulitis 1. This recommendation is based on the increasing prevalence of community-acquired MRSA (CA-MRSA) in skin and soft tissue infections.
The 2014 IDSA practice guidelines for skin and soft tissue infections further support this recommendation, listing trimethoprim-sulfamethoxazole as an appropriate treatment option for purulent cellulitis 1.
Treatment Considerations
Duration of Therapy
- Treatment duration should typically be 7-10 days, depending on clinical response 1
Monitoring
- Complete blood counts with differential and platelet count should be performed at initiation of TMP-SMX therapy and at monthly intervals to assess for hematologic toxicity 1
- Monitor for clinical improvement within 48-72 hours of starting treatment
Potential Adverse Effects
- Rash
- Gastrointestinal upset
- Bone marrow suppression (rare but serious)
- Thrombocytopenia
- Hyperkalemia
Alternative Therapies
If TMP-SMX is not tolerated or contraindicated, alternative options include:
Clindamycin: 10-13 mg/kg/dose PO every 6-8 hours, not to exceed 40 mg/kg/day 1
- Provides coverage for both β-hemolytic streptococci and CA-MRSA
- Higher risk of Clostridioides difficile-associated disease
Linezolid: 10 mg/kg/dose PO every 8 hours, not to exceed 600 mg/dose 1
- More expensive compared with other alternatives
- Reserved for severe infections or treatment failures
Important Considerations and Pitfalls
- Age restriction: TMP-SMX is not recommended for children less than 2 months of age 2
- Concomitant medications: Despite previous concerns, concomitant use of TMP-SMX and methotrexate is not contraindicated according to recent guidelines 1
- Incision and drainage: For purulent cellulitis with abscess formation, incision and drainage is the primary intervention, with antibiotics as adjunctive therapy 1
- Reassessment: The patient should be reassessed within 48-72 hours to evaluate clinical response and potentially adjust therapy 3
- Resistance concerns: In areas with high MRSA prevalence, TMP-SMX has shown higher success rates compared to beta-lactams like cephalexin 4
By following these evidence-based dosing recommendations and monitoring guidelines, you can effectively treat purulent cellulitis in this 22kg child while minimizing the risk of adverse effects and treatment failure.