Prothrombin time (PT) is a blood test that measures the time it takes for the liquid portion (plasma) of your blood to clot. It measures the function of a part of the clotting system.
A related blood test is partial thromboplastin time (PTT), which measures the function of a different part of the clotting system.
PT; Pro-time; Anticoagulant-prothrombin time; Clotting time: protime; INR; International normalized ratio
A blood sample is needed. If you are taking blood-thinning medicines, you will be watched for signs of bleeding.
Certain medicines can change blood test results.
Also, tell your provider if you're taking any herbal remedies.
When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or slight bruising. This soon goes away.
The most common reason to perform this test is to monitor your levels when you are taking a blood-thinning medicine called warfarin (Coumadin). You are likely taking this medicine to prevent blood clots.
Your provider will check your PT regularly to monitor and adjust the dose of warfarin.
You may also need this test to:
PT is measured in seconds. Most of the time, results are also given as what is called INR (international normalized ratio).
If you are not taking blood thinning medicines, such as warfarin, the normal range for your results is:
If you are taking warfarin to prevent blood clots, your provider will most likely choose to keep your INR from 2.0 to 3.0.
Ask your provider what result is right for you.
Normal value for PT ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples.
If you are not taking blood thinning medicines, such as warfarin, an INR result above 1.1 means your blood is clotting more slowly than normal. This may be due to:
If you are taking warfarin to prevent clots, your provider will most likely choose to keep your INR from 2.0 to 3.0:
A PT result that is too high or too low in someone who is taking warfarin (Coumadin) may be due to:
Your provider will teach you about taking warfarin (Coumadin) the proper way.
There is little risk involved with having your blood taken. Veins and arteries vary in size from one person to another, and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight, but may include:
This test is often done on people who may have bleeding problems. Their risk of bleeding is slightly higher than for people without bleeding problems.
Lee GM, Ortel TL. Antithrombotic therapy. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 43.
Schafer AI. Approach to the patient with bleeding and thrombosis. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 162.
Reviewed By: Mark Levin, MD, Hematologist and Oncologist, Monsey, NY. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.