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Also known as: Partial thromboplastin time; PTT; kaolin cephalin clotting time; KCCT
Formal name: Activated partial thromboplastin time
Related tests: Prothrombin time (PT), fibrinogen, thrombin time, lupus inhibitor, activated clotting time (ACT), von Willebrand screen, coagulation factors, platelet count, Heparin anti-Xa

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. To monitor heparin anticoagulant therapy

When to Get Tested?

When you have unexplained bleeding or thrombosis (a blood clot). When you are on heparin anticoagulant therapy. Sometimes as part of a pre-surgical screen

Sample Required?

A blood sample is drawn by needle from a vein in the arm

The Test Sample

What is being tested?

The activated partial thromboplastin time (aPTT or PTT) is a functional measure 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.

The Test

Common Questions

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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.