Prothrombin time (PT) is a blood test that measures the time it takes for the liquid portion (plasma) of your blood to clot. A prothrombin time test can be used to check for bleeding problems. PT is also used to check whether medicine to prevent blood clots is working. A PT test may also be called an INR test.
The prothrombin time is one of several tests that check if your blood is clotting normally. Blood clotting (coagulation) is needed to help stop bleeding. Proteins in the blood called clotting factors (coagulants) help blood become sticky and clot.
As soon as you start to bleed somewhere in or on your body, platelet cells in the blood collect around the bleeding area. The platelet cells and coagulants then react to thicken the blood and stop the bleeding. Problems in the blood, such as low levels of clotting factors or platelets, can keep blood from clotting normally and cause too much bleeding.
Clotting factors are usually made by the liver. Prothrombin is one type of clotting factor. When bleeding occurs 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, you may have a blood clotting disorder.
The 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
- Problems with the bone marrow
- Von Willebrand disease
- Problems with the immune system
- Some types of cancer, including leukemia
Why are results reported in seconds and percent?
PT test results used to be given in seconds, along with a control value. The control value usually varied somewhat from day to day because the reagents used varied. The PT value was supposed to be approximately equal to the control value. Some laboratories used to report PT values as percentages of normal activity, because the results were compared with a curve representing normal clotting time. A normal PT result was 85% to 100%. To have uniform PT results for physicians in different parts of the country and the world, the World Health Organization has recommended that PT results include the use of the international normalized ratio (INR) value. The reported INR results are independent of the reagents or methods used. Many hospitals are now reporting PT times in both absolute and INR numbers.
Other blood clotting tests:
Other blood clotting tests, such as partial thromboplastin time (PTT) and activated clotting time (aPTT), might be used if you take another type of blood-thinning medicine called heparin (instead of warfarin, for example). These tests measure other clotting factors, or they may be used to see if you are getting the right dose of heparin. Partial thromboplastin time and prothrombin time are often done at the same time to check for bleeding problems or the chance for too much bleeding in surgery.
Reasons for decreased levels (blood clotting too quickly):
Blood that clots too quickly can be caused by:
- Supplements that contain vitamin K
- High intake of foods that contain vitamin K, such as liver, broccoli, chickpeas, green tea, kale, turnip greens and products that contain soybeans
- Estrogen-containing medications, such as birth control pills and hormone replacement therapy
Reasons for increased levels (blood clotting too slow):
The prothrombin time is made longer by:
- Blood-thinning medicine, such as warfarin.
- Low levels of blood clotting factors.
- A change in the activity of any of the clotting factors.
- The absence of any of the clotting factors.
- Other substances, called inhibitors, that affect the clotting factors.
- An increase in the use of the clotting factors.
An abnormal prothrombin time is often caused by liver disease or injury or by treatment with blood thinners.
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