At a Glance
Why Get Tested?
As part of an investigation of a bleeding or thrombotic episode. To help evaluate your risk of excessive bleeding prior to a surgical procedure although numerous studies have shown that it cannot accurately estimate risk of bleeding in all patients. To monitor unfractionated heparin anticoagulant therapy
When to Get Tested?
When you have unexplained bleeding or thrombosis (a blood clot). When you are on unfractionated or intravenous (IV) heparin anticoagulant therapy. Sometimes as part of a pre-surgical screen
A blood sample is taken by needle from a vein in the arm
Test Preparation Needed?
The Test Sample
What is being tested?
What is being tested? The activated partial thromboplastin time (aPTT or PTT) is a measure of the functionality of the intrinsic and common pathways of the coagulation cascade. The body uses the coagulation cascade to produce blood clots to seal off injuries to blood vessels and tissues, to prevent further blood loss, and to give the damaged areas time to heal. The cascade consists of a group of coagulation factors. These proteins are activated sequentially along either the extrinsic (tissue related) or intrinsic (blood vessel related) pathways. The branches of the pathway then come together into the common pathway, and complete their task with the formation of a stable blood clot. When a person starts bleeding, these three pathways have to work together.
Each component of the coagulation cascade must be functioning properly and be present in sufficient quantity for normal blood clot formation. If there is an inherited or acquired deficiency in one or more of the factors, or if the factors are functioning abnormally, then stable clot formation will be inhibited and excessive bleeding and/or clotting may occur.
The aPTT test measures the length of time (in seconds) that it takes for clotting to occur when reagents are added to plasma (liquid portion of the blood) in a test tube.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed; however, eating a high fat meal prior to the blood test may cause interference with the test and should be avoided. It is important that the blood is collected from a medium to large calibre vein without the application of a tourniquet for a prolonged period of time to get an accurate result. The correct volume of blood is required for an accurate result.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.