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Warfarin Part One: Overview of Use and Dosing

May 26, 2016


Warfarin is an anticoagulant, meaning it slows down the blood-clotting process in the body and acts as a blood thinner. This helps prevent blood clots that can cause vein blockages, heart attack and stroke. Warfarin does this by interfering with the body’s use of vitamin K, a naturally occurring vitamin found in leafy green vegetables and produced by bacteria in the intestines. The human body requires vitamin K in order to make blood clot correctly, but when warfarin is introduced it prevents the liver from metabolizing vitamin K and results in a slower rate of clotting.


Warfarin is used to treat people with deep vein thrombosis, pulmonary embolism, artificial heart valves and other conditions. Some patients with a moderate to high risk of stroke benefit from Warfarin as well.

Monitoring and Dosing
The International Normalized Ratio (INR) is used to measure blood clotting time. The higher the INR, the longer it takes for the blood to clot and the “thinner” the blood is considered. The goal INR for most indications is 2.0-3.0, but for people with mechanical heart valves or recurrent clots the goal is 2.5-3.5.


The goal of warfarin therapy is to decrease the clotting tendency of blood, not to prevent clotting completely. A patient’s response to warfarin must therefore be monitored carefully through daily blood tests that determine their INR which, in turn, will determine how to adjust their next dose. The clotting factors dependent upon vitamin K have a long half life (60 hours), so it can take 5-7 days for warfarin to inhibit them and bring them down to a therapeutic level. INR should be monitored every 2-3 days when warfarin is initiated, and then every 1-4 weeks based on stability.

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