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)

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

 

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

 
 

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 / 3.5Target INR in mechanical heart valves aren't straight forward. It depends on the thrombogenicity of the valve and patient-related risk factors. Recommended target INR is either 3.0 or 3.5. Exception being target INR 2.5 in low thrombogenicity with no patient risk factors.  
  • Recurrent DVT / PE whilst anticoagulated → 3.5

 

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-coagulationExamples provided by NICE CKS include: - severe hypertension - liver disease - renal failure
• ↑ Risk of bleedingExamples provided by NICE CKS include: - INR >4 - >65 y/o
• Adherence to treatment maybe difficult

Management depends on:

• Bleeding severity
• INR