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Prothrombin time and INR

Coagulation is a critical process that allows blood to form clots to stop bleeding. This complex process prevents uncontrolled bleeding and allows wounds to heal.

Under certain circumstances, coagulation may not occur normally. Excessive coagulation increases the risk of thrombosis (blood clots), while reduced coagulation increases the risk of bleeding.

Prothrombin time is a test that measures how long it takes for blood to clot. It is primarily used to monitor warfarin (Coumadin) therapy when given as an INR (international normalized ratio).

What is the purpose of this test?
  • To monitor the effects of warfarin so that the dosage can be adjusted
  • To diagnose conditions that involve clotting factors (e.g., hemophilia)
  • To assess patients with unexplained bleeding (e.g., frequent nosebleeds or a tendency to bruise)
  • To monitor liver function
  • To perform certain preoperative assessments
How to prepare? No preparation needed.
Reference values*
  • Prothrombin time: 10 to 14 seconds
  • INR (without warfarin treatment): 0.9 to 1.2
  • INR (with warfarin treatment): 2.0 to 3.5 (target depends on the individual)
Associated tests Other tests related to blood clotting or liver health may be done at the same time, such as:
How is it done? A blood sample is drawn, usually from a vein in the arm or a simple finger prick.
How long before I get the results? Results are usually ready in a day. It may take longer to receive your results.
* These values are presented for information purposes only. As different laboratories may have different target values, the lab report usually specifies the values used for the test. It should also be noted that test results must be interpreted based on factors such as age, sex, and health condition. Values outside of this range may be considered normal for some individuals.

What does an abnormal test result mean?

Except when used for warfarin monitoring and dose adjustment, prothrombin time cannot usually be interpreted by itself. It is important to evaluate the results of other tests typically performed at the same time to determine the possible cause of abnormal values.

Possible causes of a high result:

  • Vitamin K deficiency
  • Clotting factor deficiency
  • Liver problems (e.g., hepatitis or cirrhosis)
  • Disseminated intravascular coagulation (excessive, widespread activation of the coagulation system in the blood vessels)

Factors that may affect test results

Other factors that can influence test results include:

  • Use of blood thinners (e.g., warfarin)
  • Drug interactions between warfarin and certain medications
  • Changes in diet (e.g., increase or decrease in green vegetables)
  • Alcohol use
  • Fever
  • Gastrointestinal problems (diarrhea or vomiting)

What you should know before going for this test

When you go in for a blood test or any other medical exam, it's always a good idea to bring a complete list of any prescriptions, over-the-counter medications, or natural health products you're taking. Unless otherwise advised, it's best to take your medications as usual on blood test days. If in doubt, don't hesitate to talk to your healthcare professional.

© Copyright Vigilance Santé

The patient information leaflets are provided by Vigilance Santé Inc. This content is for information purposes only and does not in any manner whatsoever replace the opinion or advice of your health care professional. Always consult a health care professional before making a decision about your medication or treatment.

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