Prothrombin Time (PT) Test


What is a prothrombin time (PT) test?

A prothrombin time (PT) test measures the time it takes for the liquid portion of your blood to clot. This liquid portion of the blood is called plasma. Clotting refers to the formation of the blood and proteins into a solid mass to stop bleeding.

Sometimes this test is called an INR (International Normalized Ratio) or ProTime test.

Why is a prothrombin time (PT) test done?

The average time it takes for blood to clot is 10 to 14 seconds. If your blood clots more slowly or more quickly than that, you may have a clotting problem. If your healthcare provider suspects this, he or she may recommend a PT test.

The most common reason for PT tests is to monitor your blood levels if you are taking the blood-thinner warfarin.

Warfarin is used to prevent blood clots. Blood clots can cause serious conditions, such as deep venous thrombosis or pulmonary embolism.

Other reasons you may be given a PT test are to:

  • Check liver function
  • Discover the cause of abnormal bruising or bleeding
  • Check for signs of bleeding disorders that can cause bleeding issues, such as hemophilia

What does a PT test check for?

A PT test checks the speed at which your blood clots.

Test Details

How is a PT test performed?

A PT test is a simple procedure. Your healthcare provider will take a blood sample from your arm (inside the elbow) or your hand. The lab will then time how quickly your blood clots, in seconds.

How do I prepare for a PT test?

Very little preparation is necessary for a PT test. However, if you are on blood thinners, you will be watched after the test to check for excessive bleeding.

A PT test is a very safe procedure. However, there is a small risk of infection, a hematoma, or feeling faint after any blood draw.

Your healthcare provider may also ask you to stop taking medicine that can affect the test results prior to your appointment. These include aspirin, heparin, antihistamines, and vitamin C.

Results and Follow-Up

What are normal results for a PT test?

Prothrombin time test results are given in a measurement called an INR (international normalized ratio).

The normal range for clotting is:

  • 11 to 13.5 seconds
  • INR of 0.8 to 1.1

For patients on warfarin, the therapeutic range is 2.0 to 3.0.

What can cause PT test results to be outside the normal range?

If your INR is above 1.1, your blood is clotting more slowly than normal. Causes of this include:

  • Bleeding disorders
  • A disorder called disseminated intravascular coagulation, in which the proteins that control blood clotting are overactive
  • Liver disease
  • Low vitamin K levels

If you are taking warfarin and have a PT result outside of the 2.0 to 3.0 range this is considered abnormal. Causes could include:

  • Not taking the proper dose of warfarin
  • Drinking alcohol
  • Taking over-the-counter medicines, such cold medicines, or vitamin supplements, that can interact with warfarin
  • Consuming foods and drinks that can interact with warfarin, such as kale, spinach, cranberry juice, and alcohol

What do I do if I have an abnormal PT test?

Your healthcare provider will advise you on the best course of action if you have an abnormal PT test. If you are not on warfarin, he or she will look into the underlying conditions that could be causing your clotting issues. If you are taking warfarin, your medicine level may need to be adjusted, or you will be instructed on the proper way to take your blood thinner.

Last reviewed by a Cleveland Clinic medical professional on 01/18/2018.


  • Lab Tests Online. Prothrombin Time and International Normalized Ratio. ( Accessed 2/22/2018.
  • Genetics Home Reference. Prothrombin Thrombophilia. ( Accessed 2/22/2018.
  • Food and Drug Administration. Prothrombin. ( Accessed 2/22/2018.

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