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Prothrombin Time

Does this test have other names?

Prothrombin time/PT, Pro time, Prothrombin time/International normalized ratio, PT/INR

What is this test?

The prothrombin time is one of several tests that evaluate whether your blood is clotting properly. Blood clotting, or coagulation, is needed to help stop bleeding. Proteins in the blood called clotting factors or coagulants help blood become sticky and clot. They change it from a liquid to a solid. 

As soon as you get a cut or the body starts to bleed, platelet cells in the blood collect around the bleeding area. The platelet cells and clotting factors then react to thicken the blood and stop the bleeding. Abnormalities in the blood, including deficiencies in clotting factors or platelets, can prevent blood from clotting normally and cause abnormal bleeding.

Clotting factors are usually produced by the liver. Prothrombin is one type of clotting factor. When bleeding occurs somewhere in the body, prothrombin quickly changes to thrombin. The prothrombin time test measures how quickly prothrombin changes to thrombin to stop the bleeding. If the prothrombin doesn't change as quickly as normal, a blood clotting disorder could be the cause. 

The prothrombin time test can find a lack of certain coagulants if the blood does not clot normally. The prothrombin time test may be used to help diagnose inherited disorders and other conditions that may affect blood clotting. These include:

  • Vitamin K deficiency

  • Deficiency in clotting factor I, II, V, VII, or X

  • Diseases of the liver

  • Hemophilia

  • Problems with the bone marrow

  • Von Willebrand disease

  • Problems with the immune system

  • Some types of cancer, including leukemia 

Why do I need this test?

Your doctor may order this test if you show symptoms of having a bleeding disorder. These symptoms can include:

  • Abnormal menstrual periods in women

  • Nosebleeds

  • Bleeding that occurs more easily

  • Bruising that occurs more easily

  • Excessive bleeding

  • Blood in the stool

Your doctor may also order this test regularly if you are taking a blood thinner such as warfarin (Coumadin) to help make sure you are getting the right dose.

You may also need this test before you have surgery or an invasive procedure to help make sure your blood is clotting properly. 

What other tests might I have along with this test?

Depending on why you are having the test, your doctor may order other blood tests along with the prothrombin time. These may include:

  • Thrombin time, or TT, test

  • Activated partial thromboplastin time, or  aPTT

These tests also measure the ability of the blood to clot normally. Other tests that measure different aspects of blood clotting, such as platelet function, may also be done.   

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

This test result is often reported in seconds. The normal clotting time is typically between 10 and 12 seconds. If your blood does not clot within that normal range, you may have a clotting or bleeding disorder.

If this test is done because you are taking an anticoagulant such as warfarin, the result is often reported as an International Normalized Ratio, which is simply a number. Doctors usually want the INR to be between 2.0 and 3.0 for people taking these drugs.   

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore. 

What might affect my test results?

A number of medications can affect the test results. These include medications that are used to help prevent blood clots, such as warfarin, aspirin, and heparin. Other medications that may affect the results include:

  • Some steroids

  • Antibiotics

  • Estrogen medications

  • Antacids

If your blood sample is not collected correctly, your test results may also be affected. 

How do I get ready for this test?

You don't usually need to prepare for this test. Your doctor will tell you whether you will need to avoid eating or drinking in the hours before the test. Your doctor may ask you to stop taking any medications that may affect your test results, especially those that prevent blood clotting. Be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.