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Prothrombin Time
Overview & Description
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:
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 After the needle is withdrawn, the puncture site is covered to prevent bleeding.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.
Preparation & Expectations
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.
Results and Values
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:
Abnormally low PT values are usually not significant. However, they may occur when a person:
Attribution
Author:David T. Moran, MD
Date Written:
Editor:Crist, Gayle P., MS, BA
Edit Date:08/30/02
Reviewer:Melinda Murray Ratini, DO
Date Reviewed:08/15/02