What is the recommended dose of Bactrim (trimethoprim/sulfamethoxazole) for a 90-year-old male patient with impaired renal function, creatinine level of 2.0, eGFR (estimated Glomerular Filtration Rate) of 31, and weighing 140lbs, with a chronic Foley catheter?

Bactrim Dosing for 90-Year-Old Male with Renal Impairment

For this patient with a creatinine clearance of approximately 31 mL/min (eGFR 31), the recommended dose is half the usual regimen: one single-strength tablet (80mg trimethoprim/400mg sulfamethoxazole) every 12 hours, or one double-strength tablet once daily. 1, 2

Rationale for Dose Adjustment

Renal Function Assessment

• With a creatinine of 2.0 mg/dL and eGFR of 31 mL/min, this patient falls into the creatinine clearance range of 15-30 mL/min category 1
• At 140 lbs (63.6 kg) and 90 years old, using the Cockcroft-Gault formula: CrCl = (140-90) × 63.6 × 1.2 / (2.0 × 88.4) ≈ 21-31 mL/min, confirming moderate-to-severe renal impairment 1

FDA-Approved Dosing Guidelines

The FDA label explicitly states for impaired renal function 2:

• Creatinine clearance 15-30 mL/min: Use half the usual regimen
• Creatinine clearance below 15 mL/min: Use not recommended

This translates to either:

• 1 single-strength tablet (80/400mg) every 12 hours, OR
• 1 double-strength tablet (160/800mg) every 24 hours

Pharmacokinetic Considerations in Renal Impairment

• Both trimethoprim and sulfamethoxazole accumulate when creatinine clearance falls below 30 mL/min 3
• The half-lives of both components increase significantly with renal dysfunction, with trimethoprim showing stronger correlation with serum creatinine (r = +0.85) 4
• Sulfamethoxazole metabolites, particularly N4-acetyl-SMZ, accumulate proportionally to creatinine levels (r = +0.92) 4
• Mean serum half-lives extend beyond the normal 8-10 hours in patients with severely impaired renal function 2

Important Clinical Caveats

Chronic Foley Catheter Consideration

• The presence of a chronic Foley catheter does not change the dosing regimen 2
• However, chronic catheterization increases risk of colonization versus true infection—ensure treatment is indicated for symptomatic infection rather than asymptomatic bacteriuria 3
• Catheter-associated bacteriuria without systemic symptoms generally does not warrant antibiotic therapy

Monitoring Requirements

• Baseline and follow-up renal function monitoring is essential 5
• Check serum creatinine and BUN within 2-3 days of initiating therapy, as acute kidney injury occurs in approximately 5.8-11.2% of patients, with higher risk in those with pre-existing renal impairment 5
• Geriatric patients show 19% lower apparent total body clearance of trimethoprim compared to younger adults 2
• Monitor for signs of drug accumulation including nausea, confusion, or worsening renal function 3, 4

Risk Factors for Adverse Effects

• This patient has multiple risk factors for trimethoprim/sulfamethoxazole-associated acute kidney injury 5:
  • Advanced age (90 years)
  • Pre-existing renal impairment (eGFR 31)
  • If hypertension or diabetes are present, risk increases substantially

Mechanism of Creatinine Elevation

• Trimethoprim (not sulfamethoxazole) causes reversible elevation in serum creatinine by competing for tubular secretion, which can increase creatinine by 0.12 mg/dL even without true renal injury 6
• This is distinct from true acute kidney injury, which typically resolves promptly after discontinuation 5
• True intrinsic renal impairment (not interstitial nephritis) accounts for the majority of cases 5

Alternative Dosing Strategy

If concerns about toxicity are high given the patient's age and borderline renal function, consider 1:

• Starting with the lower end of the dosing range
• Monitoring plasma concentrations of total sulfamethoxazole in severely impaired renal function 7
• The dosing interval in hours can be calculated as 12 times the serum creatinine level (mg/dL), with a maximum of 48 hours 4

For this patient with creatinine 2.0: dosing interval = 12 × 2.0 = 24 hours, supporting once-daily double-strength dosing as appropriate. 4