How should I dose Bactrim (trimethoprim‑sulfamethoxazole) for an adult on chronic dialysis with uncomplicated cystitis?

Bactrim Dosing for Uncomplicated Cystitis in Dialysis Patients

Recommended Dosing Regimen

For an adult on chronic hemodialysis with uncomplicated cystitis, administer one double-strength Bactrim tablet (160 mg TMP/800 mg SMX) after each dialysis session, three times weekly. 1, 2

Key Dosing Principles

• Post-dialysis timing is mandatory: Always give Bactrim immediately after completing the dialysis session, never before, as pre-dialysis dosing results in 44-57% drug removal during dialysis and leads to treatment failure. 3, 2

• Do not reduce the individual dose size: Maintain the full double-strength tablet per dose but extend the interval to three times weekly rather than daily dosing. 1, 2

• Avoid standard renal dosing charts: These apply only to non-dialysis chronic kidney disease patients and will result in underdosing in dialysis patients who require supplementation after each session due to dialytic drug removal. 1, 4

Treatment Duration

• Standard 7-day equivalent: Since dialysis patients receive medication three times weekly, continue treatment for 2-3 weeks (6-9 doses total) to approximate the standard 7-day course recommended for uncomplicated cystitis in non-dialysis patients. 5

• The European Association of Urology guidelines recommend 7 days of TMP-SMX (160/800 mg twice daily) for uncomplicated cystitis in men, which translates to approximately 2-3 weeks of thrice-weekly post-dialysis dosing. 5

Alternative Dosing Options

If using single-strength tablets (80 mg TMP/400 mg SMX):

• Give two single-strength tablets after each dialysis session, three times weekly. 2

Pharmacokinetic Rationale

• Hemodialysis removes approximately 44% of trimethoprim and 57% of sulfamethoxazole during a 4-hour session, with dialysis clearances of 38 ml/min and 42 ml/min respectively. 3

• The elimination half-life during dialysis is 6.0 hours for TMP and 3.1 hours for SMX, necessitating post-dialysis supplementation. 3

Critical Pitfalls to Avoid

• Never dose daily: The three-times-weekly post-dialysis schedule aligns with drug clearance patterns and prevents accumulation toxicity. 1, 2

• Never give before dialysis: This wastes medication and leaves the patient undertreated. 2, 4

• Do not use prophylactic dosing for active infection: Prophylactic regimens (half-dose or one single-strength tablet post-dialysis) are inadequate for treating active cystitis. 1, 2

Monitoring and Safety

• Watch for drug interactions: TMP-SMX potentiates warfarin anticoagulation and may interact with antidiabetic agents. 2, 4

• Monitor hematologic toxicity: Obtain CBC with differential at baseline and monthly during treatment. 4

• Fluid intake: Encourage adequate hydration in patients with residual urine output to minimize crystalluria risk, though this is less concerning in anuric dialysis patients. 2, 4

Special Considerations for Peritoneal Dialysis

If the patient is on peritoneal dialysis (CAPD) rather than hemodialysis:

• Start with 320 mg TMP/1600 mg SMX every 48 hours based on pharmacokinetic data showing CAPD clearance of 2.27 ml/min for TMP and 1.72 ml/min for SMX. 6

• Consider serum drug concentration monitoring to ensure adequate dosing, as data specific to peritoneal dialysis are limited. 2, 6