Warfarin (Coumadin) is the most widely used prescription anticoagulant (“blood thinner”) in the United States. On any given day, approximately three million Americans take this drug.
The most common uses for warfarin are for people who are undergoing surgery that may lead to clots which could migrate to the heart or lungs, those who have previously produced dangerous blood clots, or those with heart conditions that might lead to life-threatening clotting (e.g., atrial fibrillation or heart valve abnormalities).
Individuals who take warfarin are accustomed to the periodic blood tests that are required to keep their “bleeding time” in a therapeutic range, but few really understand how this drug works.
The Coagulation Cascade
Normally, when a person experiences an injury, a series of biochemical reactions lead to the formation of a clot that prevents uncontrolled bleeding. At the same time the clot forms, more chemical reactions keep the clot localized to prevent it from propagating inappropriately.
Two main mechanisms work in concert to produce clots in response to vessel wall injury:
- Platelets, which are small fragments of cells made in the bone marrow, react to injury by sticking together and forming a latticework where a clot can form.
- When the wall of a blood vessel is damaged, several circulating proteins (called clotting factors) undergo a stepwise series of changes—one protein activating the next in line—that eventually lead to the formation of a mature clot. It is the activation of these proteins, like a waterfall spilling from one ledge to the next, that is known as the “coagulation cascade.”
How Warfarin Interferes with Clotting
Four of the thirteen clotting factors require vitamin K to do their work. Factors II, VII, IX, and X (the factors are labeled with Roman numerals) are biologically inactive without an enzymatic process that uses vitamin K as a cofactor.
Warfarin interferes with the vitamin K-dependent enzymatic process that converts these four clotting factors from an inactive to an active state. Therapeutic doses of warfarin reduce the production of these factors by as much as 50%, which leads, in turn, to a significant drop in the speed of the clotting mechanism.
The ingestion of large amounts of vitamin K will partly counteract warfarin’s effects. This is why patients taking warfarin are cautioned to limit their intake of foods that are high in vitamin K.
Foods to Limit When Taking Warfarin
Most vegetables contain some vitamin K, but those that contain particularly high amounts are:
- Spinach
- Kale
- Collards
- Beet, mustard, and turnip greens
- Brussels sprouts
- Broccoli
- Onions
Beef and pork liver are also high in vitamin K.
Checking Your Warfarin (Coumadin) Level
Genetic differences, the use of other medications, the relative vigor of one’s liver, and other variables impact warfarin’s effectiveness in any given individual. One dose does NOT fit all.
Physicians employ two laboratory techniques to determine the proper warfarin dose for a patient:
- Prothrombin time (PT): The time it takes for blood to clot after the addition of a substance called tissue factor. A normal PT (without warfarin) is usually 12-15 seconds
- International Normalized Ratio (INR): Since tissue factor is a biological molecule, its strength varies from batch to batch. The INR is a mathematical manipulation of one’s prothrombin time based on a sensitivity standard; this compensates for the variability in different batches of tissue factor. A normal value ranges from 0.9-1.3
Doctors adjust doses of warfarin to elicit PT and INR values that are appropriate for a patient’s specific condition.
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