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prothrombin time

Alternative Names

The prothrombin time (PT) test helps measure the ability of the blood to clot normally. It is very similar to the partial thromboplastin time (PTT) test. These two tests are often ordered together.

At least 12 different proteins are involved in clotting. The PT and PTT test 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 medication
  • 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, or 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 gently 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 After the needle is withdrawn, the puncture site is covered to prevent bleeding.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 laboratory 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 normal range for a PT test depends on the standards of the laboratory. It generally falls between 10 to 15 seconds. Abnormally high PT values may occur when a person:
  • is taking blood-thinning medications, especially warfarin
  • is taking other medications, such as certain antibiotics, that interfere with the test
  • has severe liver disease
  • has disseminated intravascular coagulation (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 medications, such as birth control pills
Author: David T. Moran, MD
Reviewer: HealthAnswers Australia Medical Review Panel
Editor: Dr David Taylor, Chief Medical Officer HealthAnswers Australia
Last Updated: 1/10/2001
Potential conflict of interest information for reviewers available on request

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