Warfarin

NICE CKS Anticoagulation – oral: Scenario: Warfarin (last revised April 2024) BNF Treatment Summaries: oral anticoagulants BSH Guidelines on oral anticoagulation with warfarin - four edition

Background Information

Warfarin: vitamin K antagonist

  • Inhibit vitamin K-dependent clotting factors (II, VII, IX, X), and
  • Inhibit protein C & S

  • Embolism prophylaxis in atrial fibrillation and rheumatic heart disease
  • Embolism prophylaxis in patients with prosthetic heart valves
  • Embolism treatment of prophylaxis of VTE (venous thromboembolism)
  • TIA (transient ischaemic attacks)

Guidelines

Warfarin has an initial paradoxical pro-coagulation state, therefore it should be introduced together with heparin and continued until:

  • At least 5 days, and
  • INR ≥2 for at least 24 hours
 

To start warfarin → give warfarin AND heparin at day 0. Continue heparin until at least 5 days. Once INR ≥2, stop heparin and continue warfarin alone.

Precautions depend on the type of procedure and bleeding risk:
Procedure type Action
Low bleeding risk If INR <2.5 → proceed without stopping warfarin
High bleeding risk Stop warfarin 3-5 days prior
If anticoagulation is necessary → ensure INR <2.5 + start heparin
Dental procedures If INR <4.0 → proceed without stopping warfarin
 
 

Stop warfarin 3-5 days prior if surgical procedure carries high bleeding risk
INR cut-off for surgical procedures is generally < 2.5
INR cut-off for dental procedures is generally < 4.0

BSH Guidelines (endorsed by NICE CKS and BNF) recommend the following:
  • INR target is generally 2.5 +/- 0.5 units (note target range is no longer recommended)
  • Some exceptions
    • Mechanical mitral valve 3.0 / 3.5 
    • Recurrent DVT / PE whilst anticoagulated → 3.5
 

Some patients requiring a higher INR target due to additional risk factors for thrombosis, including mitral valve positioning, mitral stenosis and concurrent atrial fibrillation.

The INRThe ratio between an individual's prothrombin time (PT) and control samples.  is used to assess and monitor the anticoagulant effect of warfarin. 

Recommended INR measurement frequency: 
Before therapeutic range reached Daily or on alternate days
(note a meaningful INR can only be obtained 3-4 days after starting treatment)
Once within therapeutic range Twice weekly for 1-2 weeks, then
Weekly until at least 2 INR measurements within range 
Once 2 measurements within therapeutic range Measure at longer intervals, BNF recommends up to every 12 weeks

More frequent monitoring is recommended if there is:
  • ↑ Risk of over-coagulation
  • ↑ Risk of bleeding
  • Adherence to treatment maybe difficult

Management depends on:
  • Bleeding severity
  • INR

Regardless of INR:
  • Stop warfarin
  • IV vitamin K
  • Prothrombin complex concentrate (PCC)

Regardless of INR:
  • Stop warfarin (restart when INR <5.0)
  • IV vitamin K

Depends on INR:
  • INR >8.0 → stop warfarin (restart when INR <5.0) + oral vitamin K
  • INR 5.0-8.0 → withhold 1-2 doses of warfarin + ↓ subsequent doses 

References



Author: Adams Lau
Reviewer:
Last Edited: 25/02/25