Liquid Bactrim Dosing in Renal Impairment
For patients with impaired renal function taking liquid Bactrim, reduce the dose by 50% when creatinine clearance is 15-30 mL/min, and avoid use entirely when creatinine clearance falls below 15 mL/min. 1
Standard Dosing for Liquid Formulation
The liquid suspension contains 40 mg/mL sulfamethoxazole and 8 mg/mL trimethoprim when properly prepared. 1 For adults with normal renal function:
- UTIs and most infections: 20 mL (800 mg SMX/160 mg TMP) every 12 hours for 10-14 days 1
- Shigellosis: Same dose for 5 days 1
- Traveler's diarrhea: Same dose for 5 days 1
For pediatric patients over 2 months with normal renal function:
- Standard dosing: 40 mg/kg/day sulfamethoxazole and 8 mg/kg/day trimethoprim, divided every 12 hours 1
- This translates to approximately 5 mL per 10 kg body weight every 12 hours 1
Renal Impairment Dosing Algorithm
Step 1: Calculate creatinine clearance to determine the appropriate dosing regimen. 1
Step 2: Apply the following adjustments:
- CrCl >30 mL/min: Use standard dosing without adjustment 1
- CrCl 15-30 mL/min: Administer 50% of the usual dose (10 mL or 400 mg SMX/80 mg TMP every 12 hours for adults) 1
- CrCl <15 mL/min: Use is not recommended 1
Special Population: Hemodialysis Patients
For patients on hemodialysis, administer half the standard dose after each dialysis session (typically 3 times weekly). 2
- Standard-strength equivalent: 400 mg SMX/80 mg TMP (10 mL) after each dialysis 2
- Double-strength equivalent: 800 mg SMX/160 mg TMP (20 mL) after each dialysis 2
- Critical timing: Always give post-dialysis to minimize toxicity and ensure adequate levels 2
Pharmacokinetic Considerations in Renal Failure
The half-lives of both trimethoprim and sulfamethoxazole increase significantly with declining renal function. 3 In severe renal failure, the dosing interval should be extended to 12 times the serum creatinine level in mg/dL, with a maximum interval of 48 hours. 3
The hepatic metabolite N4-acetyl-sulfamethoxazole accumulates proportionally with serum creatinine elevation, which may contribute to toxicity. 3
Critical Safety Monitoring
Watch for hematologic toxicity, particularly thrombocytopenia, which correlates with higher serum trimethoprim levels and longer treatment duration in patients with renal impairment. 3
Monitor drug interactions carefully in dialysis patients, especially with anticoagulants and antidiabetic agents, as these are metabolized hepatically and may have altered pharmacokinetics. 2
Maintain adequate fluid intake when possible to prevent crystalluria, though this is less concerning in anuric dialysis patients. 2
Formulation Stability
If compounding liquid Bactrim from bulk powder (not tablets), the suspension remains stable for 90 days when stored in amber plastic bottles or syringes at either 5°C or 25°C. 4 Do not compound from commercial tablets, as they produce unacceptable formulations with persistent foam that results in inconsistent dosing. 4