What is the maximum daily dose of Bactrim (trimethoprim/sulfamethoxazole) for a 9-year-old patient weighing 46 kg?

Maximum Daily Dose of Bactrim (TMP-SMX) for a 9-Year-Old Patient Weighing 46 kg

The maximum daily dose of Bactrim (trimethoprim-sulfamethoxazole) for a 9-year-old patient weighing 46 kg should not exceed 1,600 mg sulfamethoxazole and 320 mg trimethoprim per day. 1

Dosing Calculation Based on Weight and Indication

The appropriate dosing of TMP-SMX depends on the specific indication for which it is being prescribed:

For Skin and Soft Tissue Infections (MRSA)

• Recommended dose: Trimethoprim 4-6 mg/kg/dose, sulfamethoxazole 20-30 mg/kg/dose PO every 12 hours 2
• For this 46 kg child:
  • Trimethoprim component: 184-276 mg per dose (368-552 mg daily)
  • Sulfamethoxazole component: 920-1,380 mg per dose (1,840-2,760 mg daily)
  • However, the total daily dose should not exceed 320 mg trimethoprim and 1,600 mg sulfamethoxazole 1

For PCP Prophylaxis

• Recommended dose: 750 mg/m² sulfamethoxazole with 150 mg/m² trimethoprim daily in divided doses twice a day 1
• The total daily dose should not exceed 1,600 mg sulfamethoxazole and 320 mg trimethoprim 1

For PCP Treatment

• Higher doses are used: 75-100 mg/kg/day sulfamethoxazole and 15-20 mg/kg/day trimethoprim divided every 6 hours 1
• For this 46 kg child:
  • Trimethoprim component: 690-920 mg daily
  • Sulfamethoxazole component: 3,450-4,600 mg daily
  • This exceeds the maximum daily dose, so dosing would need to be adjusted

Important Considerations

1. Weight-based dosing is critical: Inadequate dosing of TMP-SMX (<5 mg TMP/kg per day) has been independently associated with clinical failure in patients with cellulitis 3

2. Maximum dose limitations: Despite weight-based calculations, the FDA label clearly states that the total daily dose should not exceed 1,600 mg sulfamethoxazole and 320 mg trimethoprim for children 1

3. Age restrictions: TMP-SMX is contraindicated in children less than 2 months of age 1

4. Monitoring: Regular monitoring for adverse effects is recommended, especially hematologic toxicity:

   • Complete blood counts with differential and platelet count should be performed at initiation of TMP-SMX prophylaxis and at monthly intervals 2

5. Duration of therapy: For MRSA skin infections, the recommended duration is typically 5-10 days 4

Practical Dosing

For a 9-year-old weighing 46 kg, the most appropriate dosing would be:

• For most indications: One double-strength tablet (160 mg trimethoprim/800 mg sulfamethoxazole) twice daily
• This provides 320 mg trimethoprim and 1,600 mg sulfamethoxazole daily, which is at the maximum allowed daily dose 1

Potential Adverse Effects

Be aware of potential adverse effects, including:

• Gastrointestinal disturbances
• Skin rashes
• Hematologic abnormalities
• Hepatotoxicity
• Hyperkalemia

Conclusion

When prescribing Bactrim for this 9-year-old patient weighing 46 kg, adhere to the maximum daily dose of 1,600 mg sulfamethoxazole and 320 mg trimethoprim, regardless of the indication. This typically translates to one double-strength tablet twice daily.