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.

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.

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Last updated:	September 25, 2006
Author:	Jan Nissl, RN, BS
Reviewed By:	Patrice Burgess, MD - Family Medicine, Joseph O'Donnell, MD - Hematology
Editors:	Susan Van Houten, RN, BSN, MBA, Tracy Landauer