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Also known as: Activated Partial Thromboplastin Time; aPTT; APTT
Formal name: Partial Thromboplastin Time

The Test Sample

What is being tested?

The partial thromboplastin time (PTT) is a screening test that helps evaluate a person's ability to appropriately form blood clots. It measures the number of seconds it takes for a clot to form in a person's sample of blood after substances (reagents) are added. The PTT assesses the amount and the function of certain proteins called coagulation factors that are an important part of blood clot formation.

When body tissue(s) or blood vessel walls are injured, bleeding occurs and a process called hemostasis begins. Small cell fragments called platelets adhere to and then clump (aggregate) at the injury site. At the same time, a process called the coagulation cascade begins and coagulation factors are activated. Through the cascading reactions, threads called fibrin form and crosslink into a net that adheres to the injury site and stabilizes it. Along with the platelets adhering, this forms a stable blood clot to seal off injuries to blood vessels, prevents additional blood loss, and gives the damaged areas time to heal.

Each component of this hemostatic process must function properly and be present in sufficient quantity for normal blood clot formation. If there is a deficiency in one or more of these factors, or if the factors function abnormally, then a stable clot may not form and bleeding continues.

With a PTT, a person's sample is compared to a normal reference interval for clotting time. When a person's PTT takes longer than normal to clot, the PTT is considered "prolonged." A prolonged PTT may be due to a condition that decreases or creates a dysfunction in one or more coagulation factors. Less often, it may be due to a condition in which the body produces certain antibodies directed against one or more coagulation factors, affecting their function.

Sometimes a PTT may be prolonged because the person tested produces an autoantibody called an antiphospholipid antibody that interferes with the test. This type of antibody affects the results of the test because it targets substances called phospholipids that are used in the PTT. Though antiphospholipid antibodies can prolong the PTT test result, in the body they are associated with excessive clotting. A person who produces these antibodies may be at an increased risk for a blood clot. A PTT maybe used as part of an evaluation of a person with signs and symptoms of excessive clotting or antiphospholipid syndrome. (See the article on Antiphospholipid Antibodies for additional details.)

When a PTT is used to investigate bleeding or clotting episodes, it is often ordered along with a prothrombin time (PT). A health practitioner will evaluate the results of both tests to help determine the cause of bleeding or clotting episode(s).

It is now understood that coagulation tests such as the PT and PTT are based on what happens artificially in the test setting (in vitro) and thus do not necessarily reflect what actually happens in the body (in vivo). Nevertheless, they can be used to evaluate certain components of the hemostasis system. The PTT and PT tests each evaluate coagulation factors that are part of different groups of chemical reaction pathways in the cascade, called the intrinsic, extrinsic, and common pathways. (For more on this, see the article on the Coagulation Cascade.)

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, a high-fat meal prior to the blood draw may interfere with the test and should be avoided.