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Frequently asked questions about INR monitoring

INR monitoring FAQ

INR monitoring FAQ

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.

Vitamin K antagonists are oral anticoagulants (commonly called "blood thinners"), such as warfarin. These drugs 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 a vitamin K antagonist 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).

Each patient reacts to vitamin K antagonist therapy 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 patients to test according to a prescribed testing frequency to ensure they are receiving a proper dosage.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 Vitamin K antagonists, such as warfarin, are anticoagulation medications that slow down the clotting process to help keep you in a safe range.

Any condition that results in an increased risk for blood clots and is treated with a vitamin K antagonist , such as 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.

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.

The goal of monitoring your vitamin K antagonist (VKA) dose is to remain in the target range recommended by your doctor. In general, an INR target therapeutic range of 2.0 to 3.0 is appropriate4-9, however this is dependent on the indication for which you are receiving a VKA and other variables.6,7,10. Please consult and follow the instructions of your doctor regarding 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 INR is too low, you may still be at risk of excessive clotting.9 Please consult your doctor if your INR value is outside of your recommended target range.

Many things can alter your INR, including stress, missing a dose of your vitamin K antagonist therapy, 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 your medication and further slow your clotting rate, causing your INR to be too high. You may want to avoid it while on vitamin K antagonist medication. Talk to your doctor about what’s appropriate for you.

A vitamin K antagonist (VKA) works by blocking the body’s ability to use vitamin K, a necessary component in the formation of blood clots. When you’re taking this type of medication, it’s important to keep the amount of vitamin K in your diet consistent, or it may impact the effectiveness of your VKA doses.11

Green vegetables such as spinach, kale and broccoli are high in vitamin K,12 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. Heart Association. Article available from: [Accessed January 2021]
  3. Ryan, J. et al. (2008). Journal of Clinical Pharmacy and Therapeutics 33, 581-590
  4. Kirchhof et al. (2016). Eur Heart J 37, 2893–2962
  5. Zamorano et al. (2014). Eur Heart J 35, 3033–3080
  6. Mazzolia et al. (2018). Eur Heart J 39, 4208-4218  
  7. January et al. (2014). JACC 64, e1–e76
  8. Baglin et al. (2012). J Thromb Haemost 10, 698–702
  9. American Heart Association. Article available from: [Accessed January 2021]
  10. Baumgartner et al. (2017). Eur Heart J 38, 2739–2786 
  11. Agency for Healthcare Research and Quality. Article available from: [Accessed January 2021]
  12. Healthline. Article available from: [Accessed January 2021]