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Activated Clotting Time

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Also known as: ACT; Activated Coagulation Time
Formal name: Activated Clotting Time

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The Test Sample

What is being tested?

The activated clotting time (ACT) is a test that is used primarily to monitor high doses of unfractionated (standard) heparin therapy. Heparin is a drug that inhibits blood clotting (anticoagulant) and is usually given intravenously (IV) by injection or continuous infusion. High doses of heparin may be given during procedures that require that blood be prevented from clotting, such as heart bypass surgery.

In moderate doses, heparin is used to help prevent and treat inappropriate blood clot formation (thrombosis or thromboembolism) and is monitored using the partial thromboplastin time (PTT) or the heparin antifactor Xa test. Monitoring is a vital part of the anticoagulation therapy because a particular quantity of heparin can affect each person a little bit differently. If the amount of heparin administered is insufficient to inhibit the body's clotting system, blood clots may form in blood vessels throughout the body. If there is too much heparin, the patient may experience excessive, even life-threatening, bleeding.

High doses of heparin are given, for example, before, during, and for a short time after, open heart surgeries. During these operations, the patient's heart and lungs are often bypassed. Their blood is filtered and oxygenated outside of the body using mechanical devices. The blood's contact with artificial surfaces activates platelets and coagulation, initiating a sequence of steps that results in blood clot formation. A high dose of heparin prevents clot formation but leaves the body in a delicate dynamic balance between clotting and bleeding. At this level of anticoagulation, the PTT is no longer clinically useful as a monitoring tool. The PTT test involves an in vitro clotting reaction and at high levels of heparin, it will not clot. In these cases, the ACT must be used for monitoring.

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.