Warfarin

Warfarin is an anticoagulant agent that is usually used as the sodium salt.

Warfarin acts as a vitamin K antagonist so affecting the synthesis of active factors II, VII, IX, X and protein C. The therapeutic goal is to cause a partial inhibition of clotting factor synthesis, to prolong prothrombin time 2 to 4 fold.

Warfarin may be used in the treatment of established thromboembolism, for example a deep venous thrombosis diagnosed by venogram, but heparin is used until the effect of warfarin is optimal.

The warfarin dose should be titrated against measured INR, although the exact therapeutic range for the INR has not been well established.

There is no good evidence of rebound hypercoagulability on stopping warfarin.

Warfarin has many drug interactions. Look in the BNF. Note, amiodarone particularly.

In cases of warfarin overdose:

• infusion of fresh frozen plasma will provide a rapid correction of the coagulation disorder
• administration of vitamin K will correct the INR over a period of a few days but will interfere with subsequent warfarin treatment for several weeks

There is evidence for the need for regular monitoring of full blood counts in atrial fibrillation patients on warfarin (1):

• study (n=54,568) found repeated low Hb (more than 5 times) during the preceding 3 months increased risk of bleeding (OR 2.3) and stroke/TIA (OR 2.4), highlighting need for earlier, routine testing of blood cell counts to ensure timely diagnosis and treatment of anaemia

Reference:

• Helin TA, Raatikainen P, Lehto M, et al. Associations of anaemia with bleeding and thrombotic complications in patients with atrial fibrillation treated with warfarin: a registry-based nested case-control study.BMJ Open 2023;13:e071342. doi: 10.1136/bmjopen-2022-071342