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prothrombin time Alternative Names PT Definition The prothrombin time test, also called PT, helps measure how well your blood is able to clot. It is much like the partial thromboplastin time test, which is also called PTT . These two tests are often ordered together. At least 12 different proteins are involved in clotting. The PT and PTT tests each measure the function of some of these proteins. Who is a candidate for the test? This test may be done: when a person has a bleeding problem to monitor a person who is taking blood-thinning medicine before surgery to make sure a person will not bleed too much during the operation How is the test performed? A blood sample is usually taken from a vein on the forearm or hand. The skin over the vein is first cleaned with an antiseptic. Next, a strong rubber tube called a tourniquet is wrapped around the upper arm. This restricts blood flow through the veins in the lower arm and causes them to enlarge. A small needle is inserted into a vein and the tourniquet is removed. Blood flows from the vein through the needle. It is collected in a syringe or vial. After the needle is withdrawn, the puncture site is covered to prevent bleeding. The blood sample is sent to a lab for testing. What is involved in preparation for the test? Generally, no preparation is needed for this test. Because test preparation may vary, a person should ask his or her doctor for specific instructions. What do the test results mean? The healthy range for a PT test depends on the standards of the lab. It generally falls between 10 to 15 seconds. When a person is taking blood thinners, the international normalized ratio, called the INR, is calculated to determine the correct dosage of the medicine. High PT values may occur when a person: is taking blood-thinning medicines, especially warfarin is taking other medicines, such as certain antibiotics, that interfere with the test has severe liver disease has disseminated intravascular clotting, called DIC, a complex blood disorder that occurs when clotting mechanisms are activated throughout the body has certain rare, inherited bleeding disorders has a vitamin K deficiency Abnormally low PT values are usually not significant. However, they may occur when a person: has cancer has blood clots is taking certain medicines, such as birth control pills

Author: David T. Moran, MD Date Written: 04/21/00 Reviewer: Melinda Murray Ratini, DO Date Reviewed: 08/15/02 Potential conflict of interest information for reviewers available on request