Prothrombin time (PT) is a blood test that measures the time it takes for the liquid portion (plasma) of your blood to clot.
A related blood test is partial thromboplastin time (PTT).
How the Test is Performed
A blood sample is needed. If you are taking blood-thinning medicines, you will be watched for signs of bleeding.
How the Test will Feel
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.
How to Prepare for the Test
Certain medicines can change blood test results.
- Your health care provider will tell you if you need to stop taking any medicines before you have this test. This may include aspirin, heparin, antihistamines, and vitamin C.
- DO NOT stop or change your medicines without talking to your provider first.
Also tell your provider if you're taking any herbal remedies.
PT is measured in seconds. Most of the time, results are 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 PT results is:
- 11 to 13.5 seconds
- INR of 0.8 to 1.1
If you are taking warfarin to prevent blood clots, your provider will most likely choose to keep your INR between 2.0 and 3.0.
Ask your provider what result is right for you.
Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your provider about the meaning of your specific test results.
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.
Other slight risks can include:
- Fainting or feeling lightheaded
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
- Multiple punctures to locate veins
What Abnormal Results Mean
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:
- Bleeding disorders, a group of conditions in which there is a problem with the body's blood clotting process.
- Disorder in which the proteins that control blood clotting become over active (disseminated intravascular coagulation).
- Liver disease.
- Low level of vitamin K.
If you are taking warfarin to prevent clots, your provider will most likely choose to keep your INR between 2.0 and 3.0:
- Depending on why you are taking the blood thinner, the desired level may be different.
- Even when your INR stays between 2.0 and 3.0, you are more likely to have bleeding problems.
- INR results higher than 3.0 may put you at even higher risk for bleeding.
- INR results lower than 2.0 may put you at risk for developing a blood clot.
A PT result that is too high or too low in someone who is taking warfarin (Coumadin) may be due to:
- The wrong dose of medicine
- Drinking alcohol
- Taking certain over-the-counter (OTC) medicines, vitamins, supplements, cold medicines, antibiotics, or other medicines
- Eating food that changes the way the blood-thinning medicine works in your body
Your provider will teach you about taking warfarin (Coumadin) the proper way.
Why the Test is Performed
The most common reason to perform this test is to monitor your levels when you are taking a blood-thinning medicine called warfarin. You are likely taking this medicine to prevent blood clots.
Your provider will check your PT regularly.
You may also need this test to:
- Find the cause of abnormal bleeding or bruising
- Check how well your liver is working
- Look for signs of a blood clotting or bleeding disorder
Chernecky CC, Berger BJ. Prothrombin time (PT) and international normalized ratio (INR) - blood. In: Chernecky CC, Berger BJ, eds. Laboratory Tests and Diagnostic Procedures. 6th ed. St Louis, MO: Elsevier Saunders; 2013:930-935.
Ortel TL. Antithrombotic therapy. McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 23rd ed. St Louis, MO: Elsevier; 2017:chap 42.