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Unlike most medications that are administered as a fixed dose, warfarin dosing is adjusted according to the INR blood test results the dose usually changes over time. This is why those who take warfarin must have their blood tested so frequently. If the INR is too low, blood clots will not be prevented, but if the INR is too high, there is an increased risk of bleeding. It's important to monitor the INR at least once a month and sometimes as often as twice weekly to make sure the level of warfarin remains in the effective range. The INR ensures that PT results obtained by different laboratories can be compared. The INR is a standardized way of expressing the PT value. The PT is reported as the International Normalized Ratio (INR). The blood test used to measure the time it takes for blood to clot is referred to as a prothrombin time test, or protime (PT). Based on the results of the blood test, your daily dose of warfarin will be adjusted to keep your clotting time within a target range. The effect of warfarin must be monitored carefully with blood testing. The goal of warfarin therapy is to decrease the clotting tendency of blood, not to prevent clotting completely. By giving a medication that blocks the clotting factors, your body can stop harmful clots from forming and prevent clots from getting larger. Vitamin K is needed to make clotting factors and prevent bleeding. Warfarin decreases the body’s ability to form blood clots by blocking the formation of vitamin K–dependent clotting factors. The formation of a blood clot in the body is a complex process that involves multiple substances called clotting factors. Many people refer to anticoagulants as *blood thinners however, warfarin does not thin the blood but instead causes the blood to take longer to form a clot. Because warfarin interferes with the formation of blood clots, it's called an anticoagulant (PDF). Beneficial blood clots prevent or stop bleeding, but harmful blood clots can cause a heart attack, stroke, deep vein thrombosis or pulmonary embolism.

Warfarin (brand names Coumadin and Jantoven) is a prescription medication used to prevent harmful blood clots from forming or growing larger.