Why monitor warfarin use?

Staying on top of your INR helps you stay healthy

If you are taking warfarin, then INR, testing is a required part of your ongoing therapy. INR stands for “international normalized ratio." It is a standardized reporting system established by the World Health Organization (WHO). It is used as the measure of a patient's level of anticoagulation when being treated with warfarin. INR is monitored to ensure patients taking warfarin are in the therapeutic range.

Your INR results let your doctor know whether you’re taking the right amount of warfarin to keep you in range and on track. Testing is a continual commitment between you and your doctor. However, once you know your options, you’ll see how it can become a simple part of your routine.

What is warfarin?


1 (accessed January 2018).
2 (January 2018)
3 Ryan, J. et al. (2008). Journal of Clinical Pharmacy and Therapeutics 33:581-590
4 Fiumara, K. et al. (2009). Circulation 119:e220-e222.

Learn how warfarin works

Often called blood thinners, anticoagulants like warfarin (also called Coumadin®) help increase the time it takes for your blood to clot.1 For instance, if you’re taking warfarin and you cut your finger, it may take longer for the bleeding to stop than for somebody who isn’t taking an anticoagulant. 

How warfarin works

Vitamin K plays a key role in forming clotting factors, which causes the blood to clot. Warfarin blocks the formation of vitamin K-dependent clotting factors, which helps prevent and slow the formation of harmful clots. 

Who takes anticoagulants?

If you just found out from your doctor that you need to start taking warfarin, you’re not alone. Every day, millions of people worldwide take anticoagulants.2 They are commonly prescribed for or as a result of:3
• Atrial fibrillation (irregular heartbeat)
• Mechanical heart valves
• Venous thromboembolism
• Thrombophilia (tendency to cause blood clots)
• Heart attack (causing damage to heart muscle)
• Stroke

Know your number
Keeping your INR in range is key

When you’re prescribed warfarin, regular blood tests are required to ensure your blood isclotting within a target range.1 This is called INR monitoring. 

An INR test shows how quickly your blood will clot while you’re on your current dose of warfarin, which helps determine whether your dose needs to be adjusted.

Your individual target range may vary based on disease state and advising doctor’s treatments.

For more information on your range, please contact your doctor. 



1 American Heart Association, “A patient’s guide to taking warfarin,” (accessed January 2018).
2 Levi, M. et al. (2009). Seminars in Thrombosis Hemostasis 35:527-542.

Ways to test
There are three ways to test your INR

All three ways provide accurate results. However, many warfarin patients choose self-testing for the freedom and convenience it offers.

Wondering if self-testing is right for you?

Take the assessment to find out. If you are ready for self-testing, download our Get Started Guide.

Compare testing options

Think about which option works best for you and your lifestyle. All three options provide accurate INR results. 



1 The CoaguChek XS system may be used up to a maximum altitude of 14,000 feet. 

Your diet and warfarin
Learn how certain foods can impact your INR

Like other medications, warfarin can be affected by what you eat and drink. While no foods are off limits, there are three important factors you need to consider.

1. Watch your vitamin K intake. 

Produced by your body as part of the clotting process, vitamin K is also found in many green, leafy foods, such as kale, lettuce, spinach and broccoli.

High amounts of vitamin K can reverse warfarin’s blood-thinning effect.1

Does this mean you should cut back on foods high in vitamin K? No, but a consistent diet is important, as your doctor will base your recommended warfarin dose on your regular diet. Consult your doctor with dietary concerns. 

2. Avoid alcohol.

Because it immediately decreases your blood’s ability to clot, alcohol should be avoided when taking warfarin. Even if your INR levels remain within the target range, the risk of major bleeding is increased.3 What’s more, after episodes of excessive drinking, clotting can increase, putting you at greater risk of heart attack or stroke.4

3. Tell your doctor about other medications and supplements.

Because warfarin can interact with many drugs and supplements,5, 6, 7 tell your healthcare provider about everything you take, including these over-the-counter medications and supplements:

• Acetaminophen (e.g., Tylenol)
• Aspirin, including ointments/creams
• Ibuprofen (e.g., Advil)
• Naproxen (e.g., Aleve)
• Non-steroidal anti-inflammatory drugs (NSAIDs)
• Heartburn or acid reflux medicines (e.g., Nexium, Zantac)
• Cold/allergy medicines
• Antibiotics
• Herbal supplements (e.g., Ginkgo biloba, ginseng, St. John’s wort)
• Birth control pills
• Vitamin supplements with vitamin K



1 Blood Thinner Pills: Your Guide to Using Them Safely. Agency for Healthcare Research and Quality, Rockville, MD. September 2015. Available at (Accessed January 2018.)
2 USDA. (Last modified May 2016). USDA National Nutrient Database for Standard Reference, Release 28, Vitamin K Content of Selected Foods (accessed January 2018).
3 American Heart Association. “A patient’s guide to taking warfarin.” (accessed January 2018).
4 Renaud, S.C. and Ruf, J.C..(March 15, 1996). “Effects of alcohol on platelet functions.” Clinica Chimica Acta  246(1-2):77-89. (accessed January 2018).
5 Coumadin (warfarin sodium) [package insert 293US11PBS01503]. (2011). Princeton, NJ: Bristol-Myers Squibb.
6 Agency for Healthcare Research and Quality, Rockville, MD.  (August 2010). “Blood Thinner Pills: Your Guide to Using Them Safely.”, using (accessed January 2018)
7 Mayo Clinic. “Warfarin side effects: Watch for interactions.” (accessed January 2018).

Frequently asked questions about PT/INR monitoring

Frequently asked questions about PT/INR monitoring

Coagulation is the formation of blood clots inside the body. Proteins in the blood, called fibrins, and small elements in the blood, called platelets, work together to form a clot, which helps stop bleeding when you have a cut or injury.

Oral anticoagulants (blood thinners) such as warfarin or Coumadin® are drugs that help thin the blood of patients with conditions such as atrial fibrillation, thrombophilia and other diseases that increase the risk of forming blood clots. Patients taking warfarin have to sometimes make a lifelong commitment to this medication to avoid complications such as stroke or pulmonary embolism (blockage in the main lung artery).

These drugs are called vitamin K antagonists, and each patient reacts to them differently. There are also external factors that could interfere with the medication, including certain foods, stress and alcohol. That is why it is so important for anti-coagulated warfarin patients to test according to a prescribed testing frequency.1

For some people, blood clots form too easily, or they don’t dissolve properly. These clots can impede blood flowing through the body, potentially leading to heart attack or stroke.2 Anticoagulation medication such as Coumadin® or another brand of warfarin slows down the clotting process to help keep you in a safe range.

PT stands for “prothrombin time,” or the time it takes for blood to clot. INR is short for “international normalized ratio.” This is a calculation for standardizing results from PT tests. Essentially, PT/INR is a measure of whether your blood is clotting at a safe rate. You may see this referred to as PT monitoring, INR monitoring, or PT/INR.

Any condition that results in an increased risk for blood clots and is treated with warfarin (sometimes known as Coumadin® or other brand names) will require regular testing. These include:3

  • Atrial fibrillation – The most common type of arrhythmia or irregular heartbeat.
  • Venous thromboembolism (VTE) – Involving blood clots in the veins of the legs or the lungs; includes deep vein thrombosis (DVT) and pulmonary embolism (PE).
  • Mechanical heart valves – Implantable devices, especially those with manmade surfaces, can cause blood clots to form.
  • Thrombophilia – An increased tendency to form abnormal blood clots.

The goal of monitoring your warfarin dose is to remain in the target range recommended by your doctor. For most people, a result of 2.0 to 3.0 is appropriate, although those at higher risk of clotting may have a target range of 2.5 to 3.5.4 Talk to your doctor about the appropriate range for you.

If your INR is higher than the target range, blood clots may not form quickly enough, and you may experience bruising or be at increased risk of bleeding. If your PT/INR is too low, you may still be at risk of excessive clotting.4 

Many things can alter your INR, including stress, missing a warfarin dose, taking herbal supplements and other medications, and consuming certain foods and beverages, such as kale and cranberry juice. Talk to your doctor about what’s appropriate for you.

Alcohol can increase the effect of warfarin and further slow your clotting rate, causing your INR to be too high. You may want to avoid it while on warfarin.5  Talk to your doctor about what’s appropriate for you.

Warfarin works by blocking the body’s ability to use vitamin K, a necessary component in the formation of blood clots. When you’re taking warfarin, it’s important to keep the amount of vitamin K in your diet consistent, or it may impact the effectiveness of your warfarin doses.5

Green vegetables such as spinach, kale and broccoli are high in vitamin K,6 as are the following foods:

  • Tuna
  • Prunes
  • Green tea
  • Beef and chicken liver
  • Liverwurst
  • Blueberries and blackberries 

Test frequency should be determined by your doctor. 


1 Coumadin® (warfarin sodium) package insert revised October 2011.
2 American Heart Association. “What is excessive blood clotting (hypercoagulation)?” (accessed January 2018).
3 Ryan, J. et al. (2008). Journal of Clinical Pharmacy and Therapeutics 33:581-590.
4 American Heart Association. “A patient’s guide to taking warfarin,” (accessed January 2018).
5 Blood Thinner Pills: Your Guide to Using Them Safely. Agency for Healthcare Research and Quality, Rockville, MD. September 2015. Available at (Accessed January 2018.)
6 Mayo Clinic. “Warfarin diet: What foods should I avoid?” (accessed January 2018).