Prothrombin time (PT): MedlinePlus Medical Encyclopedia

P Prothrombin time (PT) is a blood test that measures the time it takes for the liquid portion (plasma) of your blood to clot.
P A related blood test is partial thromboplastin time (PTT).

T How the Test is Performed
P A blood sample is needed. If you are taking blood-thinning medicines, you will be watched for signs of bleeding.

T How to Prepare for the Test
P 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.
P Also tell your provider if you're taking any herbal remedies.

T How the Test will Feel
P 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.

T Why the Test is Performed
P 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.
P Your provider will check your PT regularly.
P 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

T Normal Results
P PT is measured in seconds. Most of the time, results are given as what is called INR (international normalized ratio).
P 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
P 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.
P Ask your provider what result is right for you.
P 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.

T What Abnormal Results Mean
P 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.
P 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.
P 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
P Your provider will teach you about taking warfarin (Coumadin) the proper way.

T Risks
P 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.
P 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

T Alternative Names
P PT; Pro-time; Anticoagulant-prothrombin time; Clotting time: protime; INR; International normalized ratio

T Patient Instructions
- Deep vein thrombosis - discharge

T References
P 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.
P 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.

T Review Date 2/7/2017
P Updated by: Todd Gersten, MD, Hematology/Oncology, Florida Cancer Specialists & Research Institute, Wellington, FL. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

