Warfarin (Coumadin) For Beginners
New to Warfarin? Here Are Key Facts
Dec 4, 2009 James Cooper
Instructions about using warfarin can come in thousands of words. It may seem that using warfarin correctly can interfere with all of life. It’s not that bad. A few baseline concepts will ease the transition.
History of Warfarin
It can’t be denied, warfarin is used as rat poison. But that’s like saying nitroglycerine is used to explode boulders. While true, that has little to do with using nitroglycerine under the tongue to relieve angina pain. Similarly, warfarin as medication, when used correctly, is not dangerous and can be life-saving.
Warfarin has a background in agriculture. It’s found in grasses and grain fields. Cattle that had excess bleeding from eating moldy hay led to its discovery. The smell of freshly mowed grass is largely from coumarins, compounds related to warfarin.
In the 1950s, the medicinal use of warfarin in humans was developed. As a medicine, it is warfarin sodium, a crystalline powder that is highly soluble in water. Trade names besides Coumadin are Jantoven, Marevan, Lawarin, and Waran.
What Warfarin Does
Warfarin interferes with the action of vitamin K. Vitamin K is essential for the production of prothrombin and other proteins necessary for blood to clot. If there is less vitamin K action, there will be less prothrombin action, and blood clotting will be inhibited. In the delicate balance between keeping blood fluid and having it clot when needed, warfarin tilts towards keeping the blood fluid. Then when it should clot, as when there is a leak in the system, the leak is less likely to be plugged with a clot; the result can be abnormal bleeding (hemorrhage) which can be severe.
How Warfarin Dose Is Determined
The intended effect of warfarin is on the clotting of blood, so a test of blood clotting monitors the effect. The test simply times how long it takes plasma to form a clot under strict laboratory conditions. This is the prothrombin time. But that’s not the key measure any more. Because different laboratories may use different reagents in the test, the key test is the ratio of the sample to a standard sample, This is the International Normalized Ratio, or INR. The INR is the number used to determine warfarin dose.
Why Are Doses Different for Same Weight People ?
About one in three people have a genetic makeup that results in slower warfarin metabolism. That means warfarin stays around longer in their body, and needs a lower dose to replace what is lost (metabolized). Tests are now available to determine who these people are, but the tests are expensive; the cost/benefit of testing is under discussion by many experts.
Some people take in more vitamin K in their diet than others. It takes more warfarin to inhibit the higher vitamin K intake.
Should I Avoid Vitamin K in My Diet?
No. The US Food and Drug Administration (FDA) approved statement is, “Do not make changes in your diet such as eating large amounts of green leafy vegetables.”
Remember, some vitamin K is necessary; that’s why it’s called a vitamin.
Brussels Sprouts and Warfarin
A small study showed that eating Brussels sprouts daily for two weeks reduced the body’s clearance of warfarin by about 15%. The volunteers in this study ate a high volume (400 grams or 4 ounces) of Brussels sprouts each day.
Source:
Bristol-Meyers product information
