Alternative Names PTT, activated partial thromboplastin time, APTT
Definition The partial thromboplastin time (PTT) test helps measure the ability of the blood to clot normally. It is very similar to the prothrombin time (PT) test. These two tests are often ordered together.
At least 12 different proteins are involved in clotting. The PTT and PT 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 After the needle is withdrawn, the puncture site is covered to prevent bleeding.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 25 to 45 seconds. Abnormally high PTT values may occur when a person:
is taking blood-thinning medications, especially heparin
is taking other medications, such as certain antibiotics, that interfere with the test
Abnormally low values are usually not important. Occasionally they are a sign of widespread cancer.
Author: David T. Moran, MD Reviewer: eknowhow Medical Review Panel Editor: Dr John Hearne Last Updated: 6/06/2005 Contributors Potential conflict of interest information for reviewers available on request
