How do you initiate warfarin (Coumadin) therapy in a patient?

Initiating Warfarin (Coumadin) Therapy

Warfarin therapy should be initiated with a dose of 2-5 mg daily for most patients, with subsequent dose adjustments based on INR results, avoiding loading doses that can lead to excessive anticoagulation. 1, 2

Initial Dosing Strategy

Starting Dose Selection

• Begin with 2-5 mg daily dosing 2
  • Use lower doses (2-4 mg) for:
    • Elderly patients
    • Debilitated patients
    • Patients with potential for greater INR response
    • Patients with genetic variations in CYP2C9 and VKORC1 enzymes
  • Standard dose (5 mg) for most other patients
• Avoid loading doses (10 mg) as they:
  • May increase risk of hemorrhagic complications
  • Do not offer more rapid protection against thrombi formation 2

Initial Monitoring Schedule

• Check INR daily after initial dose until stabilization in therapeutic range 2
• Once stabilized, gradually lengthen monitoring intervals based on:
  • Patient reliability
  • Response to therapy
  • Medication changes

Dose Adjustment Protocol

Target INR Ranges

• Most indications: INR 2.0-3.0 (target 2.5) 1, 2
• Mechanical heart valves:
  • St. Jude bileaflet valve in aortic position: INR 2.0-3.0 (target 2.5)
  • Tilting disk valves and bileaflet valves in mitral position: INR 2.5-3.5 (target 3.0)
  • Caged ball or caged disk valves: INR 2.5-3.5 (target 3.0) plus aspirin 75-100 mg/day 2

Dose Adjustment Algorithm

Use this algorithm for dose adjustments based on INR results 1:

INR	Dose Adjustment per Week
<1.5	Increase by 15%/week
1.6-1.9	Increase by 10%/week
2.0-2.9	Unchanged
3.0-3.9	Decrease by 10%/week
4.0-4.9	Hold 1 dose, then restart with dose decrease by 10%/week
≥5.0	Hold until INR is 2-3, then restart with dose decrease by 15%/week

Maintenance Phase

Dose Maintenance

• Most patients are maintained on 2-10 mg daily 2
• Adjust individual dose based on INR response
• Consider breaking scored tablets in half for flexible dosing

Monitoring Frequency

• Initial phase: Monitor INR 2-4 times per week 3
• Stable phase: Gradually extend intervals between tests
  • Maximum interval of 4 weeks is recommended (some guidelines suggest up to 12 weeks for very stable patients, but this is not widely adopted) 1, 2

Special Considerations

Managing Elevated INR

• For INR 4.0-4.9 without bleeding: Hold 1 dose, then restart with dose decreased by 10% 1, 4
• For INR ≥5.0 without bleeding: Hold warfarin and consider oral vitamin K (5 mg) 4
• For major bleeding: Administer intravenous vitamin K 10 mg and prothrombin complex concentrate 4

Missed Doses

• If a dose is missed, take it as soon as possible on the same day
• Do not double the next dose to make up for missed doses 2

Pitfalls and Caveats

1. Avoid common errors:

   • Using loading doses unnecessarily
   • Inadequate monitoring during initiation
   • Excessive dose adjustments for minor INR fluctuations

2. Drug interactions:

   • Many medications interact with warfarin
   • Always reassess INR when starting or stopping other medications 2

3. Diet considerations:

   • Maintain consistent vitamin K intake
   • Educate patients about foods high in vitamin K

4. Special populations:

   • Elderly patients require lower doses and more careful monitoring
   • Consider lower target INR (2.0-2.5) for patients >75 years with atrial fibrillation 5

By following this structured approach to warfarin initiation, you can minimize bleeding risk while achieving therapeutic anticoagulation efficiently and safely.