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Warfarin Factor Half-Lives

Warfarin is an oral anticoagulant most frequently used to control and prevent thromboembolic events. ACCP recommends that patients newly started on warfarin be bridged with LMWH or unfractionated heparin for 5 days AND until therapeutic INR is achieved.

🌟High-yield fact🌟 The presence of a therapeutic INR does not confer protection from clot formation and expansion during the first few days of warfarin therapy, so if your patient’s INR is 2.0 on day 3, it is recommended to continue bridging until day 5.

This recommendation is based on the fact that the anticoagulant activity of warfarin depends on the clearance of functional clotting factors already present in the body.

Warfarin works by inhibiting new clotting factors from forming but requires that the old factors be cleared from the body. The clearance of these clotting factors is determined by their half-lives. The earliest changes in the International Normalized Ratio (INR) are typically noted 24 to 36 hours after a dose of warfarin is administered. These changes are due to the clearance of functional factor VII, which is the vitamin K–dependent clotting factor with the shortest half-life (6 hours: after 3-5 half-lives or 24-36 hours it will be eliminated from the body). The factor with the longest half-life, prothrombin or factor II, will take 5 days to clear from the body, hence why we need to bridge for at least 5 days AND until therapeutic INR is achieved.