Prothrombin Time

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Test Overview

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.

About 12 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 heparin. Another test, the activated partial thromboplastin time (APTT) test, is a better test to find out if the right dose of heparin is being used.
• 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), 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.

Last updated:	September 15, 2008
Author:	Robin Parks, MS
Reviewed By:	Anne C. Poinier, MD - Internal Medicine, Brian Leber, MDCM, FRCPC - Hematology
Editors:	Maria Essig, Tracy Landauer