How do I transition a patient from unfractionated heparin (or low‑molecular‑weight heparin) to warfarin, including starting dose, INR monitoring, and overlap duration?

Bridging Heparin to Warfarin: Practical Protocol

Start Warfarin Early with Overlap

Begin warfarin at 5 mg daily on day 1 or 2 of heparin therapy while continuing full-dose parenteral anticoagulation, and maintain this overlap for a minimum of 5 days AND until the INR is ≥2.0 for at least 24 hours (preferably on 2 consecutive measurements). 1, 2

Why the Overlap is Critical

• The INR may reach 2.0-3.0 before adequate depletion of all vitamin K-dependent clotting factors (especially factor II/prothrombin) occurs, creating a false sense of adequate anticoagulation 1
• Factor VII depletes rapidly (within 24-48 hours), raising the INR early, but factor II has a much longer half-life (60-72 hours) and requires 4-5 days for adequate suppression 3
• Premature discontinuation of heparin based solely on INR ≥2.0 can lead to recurrent thrombosis 1

Parenteral Anticoagulation Options During Bridge

Unfractionated Heparin (UFH)

• Intravenous: 80 U/kg bolus, then 18 U/kg/hour continuous infusion 4, 1
• Adjust to maintain aPTT ratio 1.5-2.5 (corresponding to anti-factor Xa level 0.3-0.7 IU/mL) 4
• Subcutaneous alternative: 333 U/kg initial dose, then 250 U/kg twice daily 4

Low-Molecular-Weight Heparin (LMWH) - Preferred Over UFH 4

• Enoxaparin: 1 mg/kg twice daily OR 1.5 mg/kg once daily 4, 1
• Dalteparin: 200 U/kg once daily OR 100 U/kg twice daily 4, 1
• Tinzaparin: 175 U/kg once daily 4, 1

Fondaparinux

• Weight-based dosing: <50 kg = 5 mg daily; 50-100 kg = 7.5 mg daily; >100 kg = 10 mg daily 4, 1

Warfarin Dosing Strategy

Initial Dose

• Standard dose: 5 mg daily for most patients 4, 1, 5, 3
• Lower starting dose (2-4 mg): Consider for elderly patients (≥75 years), poor nutritional status, liver disease, or concurrent medications affecting warfarin metabolism 4, 1
• Avoid loading doses - they increase bleeding risk without therapeutic benefit 5, 3

Target INR

• 2.0-3.0 for venous thromboembolism 4, 1

INR Monitoring Schedule

During Overlap Phase (Days 1-7)

• Check INR daily or every other day until therapeutic range achieved 1, 5
• The INR will not rise appreciably in the first 24 hours except in rare patients requiring very low maintenance doses 5

Timing of Blood Draw When on Heparin

• Draw blood for INR at least:
  • 5 hours after last IV heparin bolus 2
  • 4 hours after stopping continuous IV heparin infusion 2
  • 24 hours after last subcutaneous heparin injection 2

When to Stop Parenteral Anticoagulation

Discontinue heparin/LMWH only after ALL criteria met: 1, 2

1. Minimum 5 days of overlap completed
2. INR ≥2.0 for at least 24 hours
3. Preferably INR ≥2.0 on 2 consecutive measurements

After Achieving Stable Therapeutic INR

• Weeks 2-3: Check INR weekly 1
• After stabilization: Every 2-4 weeks 1, 5
• Long-term stable patients: Can extend to every 4-6 weeks maximum 4, 2, 5

After Dose Adjustments

• Recheck INR within 3-7 days after any warfarin dose change 1

Common Pitfalls to Avoid

Critical Errors

• Never stop heparin at day 3-4 just because INR is 2.0 - you need the full 5-day overlap minimum 1, 2
• Never use loading doses of warfarin (e.g., 10 mg × 2-3 days) - this dramatically increases bleeding risk without faster therapeutic effect 5, 3
• Never check INR immediately after heparin bolus - wait appropriate intervals to avoid falsely elevated results 2

Special Populations Requiring Caution

Renal impairment (CrCl <30 mL/min):

• LMWH accumulates and is contraindicated 4
• Use UFH instead with aPTT monitoring 1

Cancer patients:

• Higher risk of both recurrent VTE and bleeding 4, 1
• LMWH monotherapy preferred over warfarin when possible 1
• More frequent INR monitoring needed due to chemotherapy drug interactions 1

Elderly patients (≥75 years):

• Start warfarin at 2-4 mg daily rather than 5 mg 4, 1
• Require approximately 1 mg/day less than younger patients for same INR 4

Practical Timeline Example

Day 1: Start UFH or LMWH + warfarin 5 mg (or 2-4 mg if elderly)
Day 2: Continue both, check INR
Day 3: Continue both, check INR
Day 4: Continue both, check INR
Day 5: Continue both, check INR - if ≥2.0, recheck next day
Day 6: If INR ≥2.0 on consecutive days AND 5 days completed, stop heparin/LMWH 1, 2

Most patients achieve therapeutic INR by days 6-7 with this protocol 1, 6