What antibiotics can be used to treat a urinary tract infection in a patient taking warfarin (anticoagulant)?

Antibiotic Selection for UTI in Warfarin Patients

Nitrofurantoin is the preferred first-line antibiotic for uncomplicated lower urinary tract infections in patients taking warfarin, as it has minimal drug-drug interaction risk and maintains excellent efficacy against common uropathogens. 1

Lower Urinary Tract Infection (Uncomplicated Cystitis)

First-Line Options

• Nitrofurantoin 100 mg PO every 6 hours for 5 days is the optimal choice for warfarin patients with uncomplicated cystitis 1, 2

  • Maintains 95.6% susceptibility against E. coli with only 2.3% resistance rates 2
  • Minimal systemic absorption reduces drug interaction potential with warfarin 3, 4
  • Achieves high urinary concentrations while sparing fluoroquinolones 4

• Amoxicillin-clavulanic acid 875/125 mg PO twice daily is an acceptable alternative 1, 5

  • However, be aware that beta-lactams including ceftriaxone have documented cases of INR elevation when combined with warfarin 6
  • Monitor INR more frequently (within 3-4 days) if using this option 6

Avoid in Warfarin Patients

• Trimethoprim-sulfamethoxazole should be avoided despite guideline recommendations for general populations 1

  • Significantly potentiates warfarin effect through CYP2C9 inhibition
  • High resistance rates (29%) further limit utility 2

• Fluoroquinolones (ciprofloxacin, levofloxacin) should be reserved for severe infections only 1, 7

  • FDA warnings regarding serious adverse effects limit first-line use 1
  • Resistance rates approaching 24% for E. coli 2

Upper Urinary Tract Infection (Pyelonephritis/Prostatitis)

Mild-to-Moderate Disease

• Ceftriaxone or cefotaxime IV are preferred over fluoroquinolones for patients requiring parenteral therapy 1
  • Critical caveat: Ceftriaxone has documented cases causing INR elevation to 10.74 and 16.99 in warfarin patients 6
  • Monitor INR within 4 days of initiating ceftriaxone and consider holding one warfarin dose if INR >5 6
  • Have phytonadione 5 mg available for reversal if needed 6

Severe Disease

• Ceftriaxone/cefotaxime remain first-line with intensive INR monitoring 1
• Amikacin is an alternative for carbapenem-sparing in resistant organisms 1
  • Less drug interaction concern with warfarin compared to beta-lactams

Special Considerations for VRE UTI

If vancomycin-resistant enterococcus is suspected or confirmed:

• Nitrofurantoin 100 mg PO every 6 hours for uncomplicated UTI 1
• Fosfomycin 3 g PO single dose as alternative 1
• High-dose ampicillin (18-30 g IV daily) or amoxicillin 500 mg PO/IV every 8 hours for uncomplicated cases 1, 5

Monitoring Protocol for Warfarin Patients

• Baseline INR before starting antibiotics 6
• Repeat INR 3-4 days after antibiotic initiation, especially with beta-lactams 6
• Weekly INR monitoring during antibiotic course and for 1 week after completion 6
• Educate patients to report any signs of bleeding or unusual bruising immediately 6

Treatment Duration

• Uncomplicated cystitis: 5 days for nitrofurantoin 1
• Pyelonephritis: 7 days for beta-lactams 1
• Fluoroquinolones (if used): 5-7 days for pyelonephritis 1