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Prothrombin Time and INR

Prothrombin time (PT) is a blood test that measures how long it takes 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. INR (international normalized ratio) stands for a way of standardizing the results of prothrombin time tests, no matter the testing method. So your doctor can understand results in the same way even when they come from different labs and different test methods. Using the INR system, treatment with blood-thinning medicine (anticoagulant therapy) will be the same. In some labs, only the INR is reported and the PT is not reported.

Blood clotting factors are needed for blood to clot (coagulation). Prothrombin, or factor II, is one of the clotting factors made by the liver. Vitamin K is needed to make prothrombin and other clotting factors. Prothrombin time is an important test because it checks to see if five different blood clotting factors (factors I, II, V, VII, and X) are present. 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.

Another blood clotting test, called partial thromboplastin time (PTT), might be used if you take another type of blood-thinning medicine called heparin. This test measures other clotting factors. 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.

Why It Is Done

Prothrombin time (PT) is measured to:

  • Find a cause for abnormal bleeding or bruising.
  • Check to see if blood-thinning medicine, such as warfarin (Coumadin), is working. If the test is done for this purpose, a PT may be done every day at first. When the correct dose of medicine is found, you will not need so many tests.
  • Check for low levels of blood clotting factors. The lack of some clotting factors can cause bleeding disorders such as hemophilia, which is passed in families (inherited).
  • Check for a low level of vitamin K. Vitamin K is needed to make prothrombin and other clotting factors.
  • Check if it is safe to do a procedure or surgery that might cause bleeding.
  • Check how well the liver is working. Prothrombin levels are checked along with other liver tests, such as aspartate aminotransferase and alanine aminotransferase.
  • Check to see if the body is using up its clotting factors so quickly that the blood can't clot and bleeding does not stop. This may mean the person has disseminated intravascular coagulation (DIC).

WebMD Medical Reference from Healthwise

Last Updated: September 06, 2012
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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