Quick Guide to Warfarin Dosing

If you've been treated for deep venous thrombosis, then you can expect at least three months of continued anticoagulants or blood-thinning medication after you leave the hospital.

Deep venous thrombosis is a dangerous condition where your body forms a clot, typically in the leg, that can throw other clots which potentially clog up the circulation to your lungs.

This deadly event is known as pulmonary embolism. With continued anticoagulant therapy after you leave the hospital, you minimize the risk of reforming such deadly blood clots. Of note, anticoagulant therapy is not only given with deep venous thrombosis. For example, if you are currently being treated for stroke or some hypercoagulable condition, you can expect a lifetime of anticoagulation.

Anticoagulation can be maintained by several medications including low-molecular-weight heparin (subcutaneous injection), fondaparinux (subcutaneous injection) or the oral Xa inhibitors like dabigatran.  In this article, we'll focus on warfarin (Coumadin) which is commonly available as an oral medication. When choosing an anticoagulant therapy, please keep in mind that options do exist, and your physician can further discuss these options with you.

How Warfarin Works

Warfarin interferes with the hepatic synthesis of clotting factors which are vitamin K-dependent, thus messing up the process of coagulation and preventing the formation of any new clots.

Until it settles in, warfarin is paired with a parenteral or injectable anticoagulant like Lovenox (enoxaparin injection). 

Warfarin is available in both oral and intravenous preparations—most people take oral warfarin. People are started on about 5 mg of warfarin a day for the first few days. Warfarin dose is then adjusted in order to keep INR levels, measures of coagulation status that I'll touch on in a bit, therapeutic.

If interested, the website www.warfarindosing.org offers a free warfarin-dose calculator.

Who Needs Higher Doses?

Higher doses of warfarin may be required for the following patient populations:

• African American
• Obese
• Hypothyroid
• Alcohol dependent

Who Needs Lower Doses?

Conversely, the following patient populations should receive lower doses of warfarin:

• Elderly
• Asian
• Hyperthyroid
• Heart failure
• Liver disease
• History of major surgery
• Polymorphisms in CYP2C9 or VKORC1 genes

In addition to the above patient population characteristics, warfarin dosage is also adjusted depending on the medications that you're taking. For example, phenytoin increases the action of warfarin and thus lowers INR levels.

The laboratory measure INR (international normalized ratio) is used to determine your coagulation status and adjust your dosage of warfarin. In most people, normal INR levels range from 0.8 to 1.2. People on warfarin are typically maintained at an INR level between 2 and 3, meaning that these patients are significantly more anticoagulated (have thinner blood) than average people. While receiving anticoagulation, INR levels should be regularly monitored by your physician.

How Diet Affects Dosage

Because warfarin is a vitamin K antagonist, a diet rich in vitamin K can lower your INR levels.

Specifically, green and leafy vegetables tend to be high in vitamin K. On the other hand, potatoes, fruits, and cereals are low in vitamin K. You can still continue eating nutritious portions of green and leafy vegetables, however, try to remain consistent in your daily consumption so that your INR levels don't fluctuate.

Risks

Adverse effects of warfarin therapy are typically limited to nausea, cramps and so forth. However, warfarin increases your ​risk for hemorrhage or bleeding which can be quite serious (think a brain bleed). In case of serious hemorrhage on account of treatment with warfarin, physicians can administer vitamin K to help coagulate or clot the blood.

People who have a history of hemorrhage should be careful when taking warfarin. Furthermore, when taking warfarin, it's best to use an electric razor and electric toothbrush to limit the risk of bleeds. Also, be sure to inform your dentist that you're on warfarin before any dental work is done.

Because warfarin can interact with a wide range of drugs and throw your INR levels in whack, it's best that you avoid over-the-counter medications or herbal supplements while on this medication.  

If you or a loved one need to take warfarin for deep venous thrombosis, stroke, or a hypercoagulable condition, please remember that anticoagulant therapy with this drug is an involved process. To establish and maintain proper anticoagulation status, you will need to work closely with your physician and preferably a dietitian, too. Please remember to remain vigilant, get your INR levels checked regularly, work with your healthcare team, and closely watch your diet. 

Sources:

Fogarty PF, Minichiello T. Disorders of Hemostasis, Thrombosis, & Antithrombotic Therapy. In: Papadakis MA, McPhee SJ, Rabow MW. eds. Current Medical Diagnosis & Treatment 2015. New York, NY: McGraw-Hill; 2014.

Mosby's Drug Reference for Health Professions, Second Edition. Elsevier in 2010.