Warfarin (brand names Coumadin and Jantoven) is a prescription medication used to prevent harmful blood clots from forming or growing larger. Beneficial blood clots prevent or stop bleeding, but harmful blood clots can cause a stroke, heart attack, deep vein thrombosis, or pulmonary embolism. Because warfarin interferes with the formation of blood clots, it is called an anticoagulant. 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.
How Does Warfarin Work?
The formation of a clot in the body is a complex process that involves multiple substances called clotting factors. Warfarin decreases the body’s ability to form blood clots by blocking the formation of vitamin K–dependent clotting factors. Vitamin K is needed to make clotting factors and prevent bleeding. Therefore, by giving a medication that blocks the clotting factors, your body can stop harmful clots from forming and prevent clots from getting larger.
WHAT IS THE PT/ INR AND HOW IS IT MEASURED?
The INR is the International Normalised Ratio, a measure of how fast blood clots and this evaluates the effectiveness of warfarin in thinning your blood.
Your INR will be measured usually by pricking your finger to obtain a small droplet of blood which is put onto a special strip which can be analysed by a hand-held point-of-care device.
- In people who are not taking a blood-thinning medicine, blood clots with INR of around 1.0. To reduce the risk of a stroke in atrial fibrillation the blood needs to be 2-3 times thinner than normal.
- This means that the blood takes 2-3 times longer to clot.
- If you have atrial fibrillation your target INR range will be 2.0 to 3.0.
- If your blood is too thick (INR less than 2.0), then you are still at increased risk of having a stroke (caused by a clot).
- If your blood is too thin (INR greater than 3.0), then this increases your risk of bleeding.
- Some people attend a clinic (either at the hospital or their GP or health centre) to have their INR monitored. Other people self-monitor and/or self-manage their INR at home but this is not suitable or possible for all patients. If you wish to self-manage your INR you need to discuss this with your doctor.
- When you first start warfarin it may be necessary to have your INR monitored every week but once your INR becomes therapeutic (in the INR range of 2.0 to 3.0) and is stable in that range, when you will only need to have INR checked every 4 to 6 weeks. Remember, it is very important to keep your INR in the recommended range of 2.0 to 3.0.
WHAT CAN I DO TO HELP TO KEEP MY PT/INR IN THE DESIRED RANGE?
To help Warfarin work effectively, it is important to keep your vitamin K intake as consistent as possible. Sudden increases in vitamin K intake may decrease the effect of Warfarin. On
the other hand, greatly lowering your vitamin K intake could increase the effect of Warfarin.
To keep INR / PT stable and within the recommended range, it is important to:
- take the correct dose of Warfarin at the same time every day
- have your INR / PT checked regularly
- keep your vitamin K intake consistent from day to day
To help make it easier to keep your intake of vitamin K consistent:
- limit intake of foods considered “high” in vitamin K to no more than 1 serving each day
- limit intake of foods “moderately high” in vitamin K to no more than 3 servings each day
- report any significant changes in your diet or your weight to your doctor
In other words,
- Watch how often you eat foods high in vitamin K.
- Watch how much you eat of foods high in vitamin K
VITAMIN K AND WARFARIN
Green leafy vegetables are among the best food sources of vitamin K. The average intake of vitamin K for most adults in the U.S. is 70 to 80 micrograms (mcg) per day. The Daily Value for vitamin K, an estimate of daily need, is 80 micrograms. The Percent Daily Values (%DV), listed on the tables below, help consumers determine if a food contains a little or a lot of a specific nutrient.
As indicated below, it is important to limit intake of foods that provide more that 60% of the Daily Value for vitamin K to help keep INR / PT in the desired range.
Foods high in Vitamin K (more than or equal to 200% DV)
Eat no more than 1 serving per day
Food Serving size % Daily Value
Kale, fresh, boiled 1/2 cup 660
Spinach, fresh, boiled 1/2 cup 560
Turnip greens, frozen, boiled 1/2 cup 530
Collards, fresh, boiled 1/2 cup 520
Swiss chard, fresh, boiled 1/2 cup 360
Parsley, raw 1/4 cup 300
Mustard greens, fresh, boiled 1/2 cup 260
Foods moderately high in Vitamin K (60 to 199% DV)
Eat no more than 3 servings per day
Food Serving size % Daily Value
Brussels sprouts, frozen, boiled 1/2 cup 190
Spinach, raw 1 cup 180
Turnip greens, raw, chopped 1 cup 170
Green leaf lettuce, shredded 1 cup 125
Broccoli, raw, chopped 1 cup 110
Endive lettuce, raw 1 cup 70
Romaine lettuce, raw 1 cup 70
Iceberg lettuce, red cabbage, asparagus, and soybean oil are often reported as being high in vitamin K. They contain much smaller amounts than foods listed in the tables above. These, and other foods and beverages not listed in the tables above (including coffee and tea), may be consumed as desired. Food Values are from the U.S. Department of Agriculture, Agricultural Research Service. 2003. USDA National Nutrient Database for Standard Reference, Release 16. Nutrient Data Laboratory Home Page, http://www.nal.usda.gov/fnic/foodcomp
WHAT ELSE SHOULD YOU KNOW ABOUT WARFARIN?
Alcohol intake greater than 3 drinks daily can increase the effect of Warfarin. However, some medical doctors advise those taking Warfarin to avoid all alcoholic beverages. Check with your doctor about this issue.
One drink = 5 ounces wine
12 ounces beer
1 1/2 ounces liquor
DIETARY SUPPLEMENTS AND HERBAL MEDICATIONS
Many dietary supplements can alter the INR/ PT. Dietary supplements known to affect the INR/PT include: arnica, bilberry, butchers broom, cat’s claw, dong quai, feverfew, forskolin, garlic, ginger, ginkgo, horse chestnut, insositol hexaphosphate, licorice, melilot (sweet clover) pau d’arco, red clover, St. John’s wort, sweet woodruff, turmeric, willow bark, and wheat grass.
Much is unknown about dietary supplements. The safest policy is for individuals on Warfarin to avoid all dietary supplements unless their physicians approve. This includes any vitamin/mineral supplements that list vitamin K on the label. If they are taken regularly on a daily basis, they pose less of a problem than if taken off and on.
VITAMIN E SUPPLEMENTS
Evidence suggests that vitamin E has blood-thinning effects. Vitamin E intakes above 1,000 International Units (IU) per day may increase the risk of excess bleeding. Research suggests that doses up to 800 IU may be safe for individuals on Coumadin®, but the evidence is not conclusive. It is best for those taking Warfarin to ask their physicians about taking Vitamin E supplements.
Some antibiotics can either lower vitamin K levels in the body or interfere with the activity of Warfarin. Check with your physician or pharmacist about whether you will need to adjust your vitamin K intake or Warfarin dose when you take antibiotics.
What are three important things to remember about Warfarin and vitamin K?
- Warfarin is a very important drug for you. Follow the prescription exactly, and keep your follow-up appointments for blood tests such as the INR/PT.
- Warfarin interacts with vitamin K in your body, so you need to keep vitamin K intake constant from day to day. It is also important to avoid herbal products and dietary supplements that may affect vitamin K and Warfarin unless approved by a qualified health care provider.
- Post the phone numbers of your doctor, pharmacist, and registered dietitian for ready reference when you have a question or concern about Warfarin, vitamin K, and your INR / PT.