What antibiotic should be started for a 65-year-old woman with a urinary tract infection (UTI), impaired renal function, and hypertension, who is currently taking amlodipine, insulin (Lantus) (glargine), and lisinopril?

Treatment of Urinary Tract Infection in a Patient with Impaired Renal Function

Trimethoprim-sulfamethoxazole is the most appropriate antibiotic for this 65-year-old woman with a urinary tract infection, impaired renal function, and hypertension. 1, 2

Clinical Assessment

This patient presents with:

• Asymptomatic bacteriuria with >100,000 CFU of E. coli in urine culture
• Stable chronic kidney disease (creatinine 2.4, baseline 2.3)
• Hypertension controlled on amlodipine and lisinopril
• Hyperkalemia (K+ 5.8)
• No urinary symptoms or discomfort
• Recent colorectal cancer surgery

Antibiotic Selection Algorithm

First-line Considerations

1. Trimethoprim-sulfamethoxazole (TMP-SMX) - 160/800 mg twice daily for 7 days

   • Effective against E. coli
   • Appropriate for patients with impaired renal function with proper monitoring
   • Recommended by EAU guidelines for UTI treatment in patients with comorbidities 1

2. Ciprofloxacin - Not recommended as first-line due to:

   • Increasing resistance rates
   • Risk of adverse effects in elderly patients
   • Should be reserved for more severe infections or when first-line agents cannot be used 1

3. Amoxicillin - Not recommended due to high rates of E. coli resistance

4. Nitrofurantoin - Contraindicated in patients with creatinine clearance <30 mL/min (this patient likely falls in this category)

Rationale for TMP-SMX Selection

1. Efficacy: TMP-SMX is effective against E. coli, which is the identified pathogen in this case 1, 2

2. Renal Considerations:

   • While dose adjustment is needed in severe renal impairment, TMP-SMX can be used with appropriate monitoring in patients with stable CKD 3
   • The patient's renal function is stable (creatinine at baseline)

3. Drug Interactions:

   • TMP-SMX may increase potassium levels, which requires monitoring given the patient's existing hyperkalemia (5.8)
   • However, with proper monitoring, this can be managed

4. Current Medications:

   • Lisinopril: May contribute to hyperkalemia, but patient has been stable on this medication 4, 5
   • Amlodipine: Generally well-tolerated in renal impairment 6
   • Insulin glargine: No significant interaction with proposed antibiotic

Implementation Plan

1. Dosing: TMP-SMX 160/800 mg twice daily for 7 days

   • Consider reducing to once daily if renal function worsens

2. Monitoring:

   • Serum potassium levels within 48-72 hours of starting treatment
   • Renal function tests after 3-4 days of treatment
   • Blood glucose levels (TMP-SMX can rarely cause hypoglycemia in patients with renal impairment) 3

3. Patient Education:

   • Complete full course of antibiotics
   • Report any new symptoms (nausea, rash, unusual fatigue)
   • Maintain adequate hydration

Special Considerations

• Hyperkalemia Risk: The combination of TMP-SMX and lisinopril increases risk of hyperkalemia, but with the patient's stable renal function and proper monitoring, this risk can be managed
• Asymptomatic Bacteriuria: While generally not treated in most adults, treatment is warranted in this post-surgical patient with recent colorectal cancer surgery to prevent potential complications
• Fluoroquinolones: Should be avoided as first-line therapy due to risk of adverse effects in elderly patients and increasing resistance rates

By using TMP-SMX with appropriate monitoring, this patient can receive effective treatment for her UTI while minimizing risks associated with her renal impairment and other comorbidities.