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Prothrombin time (PT)
Alternative namesThis is a test that measures the clotting time of plasma (the liquid portion of the blood).
How the test is performed
Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.
A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.
How to prepare for the testThe health care provider may advise you to stop taking drugs that can affect the test (see "Special Considerations").
For infants and children:
The preparation you can provide for this test depends on your child's age, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics:
How the test will feelWhen the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performedThe PT is a broad screening test for many types of bleeding disorders. It evaluates blood clotting disorders, usually bleeding.
Normal ValuesThe normal range is 11 to 13.5 seconds ("normal" varies somewhat in different labs).
For a person on full anticoagulant therapy, the PT should be 2 to 3 times the laboratory "control" value.
What abnormal results meanIncreased PT times may indicate:
This test may be performed under many conditions in the evaluation of a wide variety of disorders.
What the risks are
Coagulation (blood clotting) results from a sequence of reactions involving several proteins known as coagulation factors. Some of these factors have other names. For example, Factor I is also called fibrinogen, Factor II is prothrombin, and Factor XII is Hageman factor. The liver produces these proteins and secretes them into the blood. In addition, vitamin K is important to blood clotting because your body converts it into prothrombin.
Some people take warfarin to keep their blood from clotting. Warfarin inhibits prothrombin, thus interrupting the clotting cascade. Because of the link between vitamin K and prothrombin, people who take warfarin need to have consistent levels of vitamin K in their diet, as instructed by their doctor.
Coagulation begins when some of the coagulation factors contact damaged tissue. Each factor reaction triggers the next reaction, in a cascade. The final product of the coagulation cascade is the blood clot .
Update Date: 6/1/2003Marcia S. Brose, M.D., Ph.D., Division of Hematology/Oncology, Hospital of the University of Pennsylvania, Philadelphia, PA. Review provided by VeriMed Healthcare Network.
Last updated: Tue, 06 Jan 2009 00:20:03 GMT