What is the recommended dose of Bactrim (sulfamethoxazole and trimethoprim) for a 70 kg patient with potential impaired renal function?

Bactrim Dosing for a 70 kg Patient

For a 70 kg patient with normal renal function, administer Bactrim 160/800 mg (one double-strength tablet) twice daily, but you must adjust the dose to half (80/400 mg twice daily) if creatinine clearance is 15-30 mL/min, and avoid use entirely if CrCl is below 15 mL/min. 1

Standard Dosing by Indication

The appropriate dose depends critically on the clinical indication and renal function:

For Urinary Tract Infections

• Standard dose: 160/800 mg (1 DS tablet) twice daily for 10-14 days 1
• For uncomplicated cystitis in women, this same dose is given for only 3 days with 90-100% clinical cure rates 2
• Critical caveat: Men require 7-14 days of therapy; the 3-day regimen used in women is inadequate and leads to treatment failure 2

For Pneumocystis Jiroveci Pneumonia (PCP)

• Treatment dose: For a 70 kg patient, administer 5,250-7,000 mg sulfamethoxazole with 1,050-1,400 mg trimethoprim per 24 hours, divided every 6 hours for 14-21 days 1
  • This translates to approximately 2-2.5 DS tablets every 6 hours (8-10 DS tablets daily) 1
• Prophylaxis dose: 160/800 mg (1 DS tablet) once daily 1
• Alternative prophylaxis: 160/800 mg three times weekly on consecutive days 3, 4

For MRSA Skin/Soft Tissue Infections

• Dose range: 320-640 mg TMP with 1,600-3,200 mg SMZ total daily (1-2 DS tablets twice daily) 4

For Traveler's Diarrhea

• Standard dose: 160/800 mg (1 DS tablet) twice daily for 5 days 1

Mandatory Renal Dose Adjustments

This is where most dosing errors occur—you must calculate creatinine clearance before prescribing:

CrCl >30 mL/min

• Use standard dosing regimens 2, 1

CrCl 15-30 mL/min

• Reduce to half-dose: 80/400 mg (1 single-strength tablet) twice daily 2, 1
• For prophylaxis: 500 mg three times weekly after dialysis in hemodialysis patients 3, 4

CrCl <15 mL/min

• Use is not recommended 1, 5, 6
• Peritoneal dialysance is very low (5.1 mL/min for TMP, 1.2 mL/min for SMZ), making dosing unpredictable 7

Critical Monitoring in Renal Impairment

• Obtain baseline creatinine clearance calculation before initiating therapy 2
• Monitor serum creatinine and BUN 2-3 times weekly during therapy 2
• Monitor electrolytes regularly, as trimethoprim blocks potassium excretion and causes hyperkalemia 2
• Common pitfall: Elderly patients often have reduced creatinine clearance despite "normal" serum creatinine due to decreased muscle mass—always calculate CrCl, don't rely on serum creatinine alone 8

Special Populations

Elderly Patients (Age >65)

• Trimethoprim peak concentrations are 30% higher and AUC is 44% larger in elderly patients compared to younger adults 8
• Renal clearance of TMP is reduced by approximately 65% in elderly patients (19 vs 55 mL/h/kg) 8
• Steady-state plasma concentrations during continuous dosing are 2-3 times higher than after a single dose 8
• Risk consideration: Trimethoprim increases risk for acute kidney injury and hyperkalemia, especially when used with ACE inhibitors, ARBs, or potassium-sparing diuretics 3

Obese Patients (BMI ≥30)

• For doses >8 mg/kg/day TMP, calculate adjusted body weight (ABW): ABW = IBW + ([TBW-IBW] × 0.4) 3
• Use ABW for dosing calculations to avoid toxicity 3

When NOT to Use Bactrim

You must verify local resistance patterns before prescribing empirically:

• Do not use empirically when local E. coli resistance exceeds 20% 2
• Clinical cure rates drop from 84% (susceptible organisms) to only 41% (resistant organisms) 2
• Absolute contraindications:
  • Pregnancy at term 4
  • G6PD deficiency (risk of hemolytic anemia) 4
  • Concurrent methotrexate at treatment doses (risk of severe bone marrow suppression) 4

Alternative First-Line Agents

When Bactrim cannot be used for UTI:

• Nitrofurantoin monohydrate/macrocrystals: 100 mg twice daily for 5 days (90% clinical cure rate) 3, 2
• Fosfomycin trometamol: 3 g single dose 2
• Pivmecillinam: 400 mg three times daily for 3-5 days 2

Key Clinical Pitfalls to Avoid

1. Never use 3-day regimens in men—this leads to treatment failure 2
2. Never fail to adjust dose when CrCl <30 mL/min—this significantly increases toxicity risk including hyperkalemia and crystalluria 2
3. Never use empirically when local resistance exceeds 20%—treatment failure is the expected outcome 2
4. Never ignore drug interactions with warfarin (reduce dose by 50%), digoxin (reduce by 30-50%), or methotrexate 3, 4
5. Never prescribe without calculating CrCl in elderly patients—serum creatinine alone is misleading 8