At a Glance
Why Get Tested?
When to Get Tested?
When you are receiving high dose heparin to prevent clotting during surgical procedures such as a cardiopulmonary bypass; when heparin levels are too high to allow monitoring with a PTT test and/or when a rapid result is necessary to monitor treatment
A blood sample taken from a vein in your arm
Test Preparation Needed?
The Test Sample
What is being tested?
ACT is a blood clotting test that is used primarily to monitor high doses of heparin anticoagulant therapy. Heparin is a blood clot inhibitor that is usually given either intravenously (IV) or by subcutaneous (under the skin) injection. In moderate doses it is used to help prevent and treat inappropriate blood clot formation (thrombosis or thromboembolism) and is monitored using the partial thromboplastin time (PTT) test or the anti-Xa test. Monitoring is a vital part of the anticoagulation therapy because a particular quantity of heparin will affect each person a little 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 before, during, and for a short time after, open heart surgery. 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 normally 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 (test-tube) clotting reaction and at high levels of heparin it will not clot.
The ACT is a rapid test that can be performed at a person's bedside prior to surgery or other medical procedures and in or near the operating room at intervals during and immediately after the surgery. Like the PTT, it measures the inhibiting effect that heparin and other antithrombotic medications have on the body’s clotting system, not the actual level of heparin in the blood. ACT testing allows relatively rapid changes in heparin infusion, helping to achieve and maintain a constant level of anticoagulation. Once surgery is complete and the patient has been stabilised, heparin doses are usually decreased.
The sensitivity of the ACT test to heparin depends on the method used. Some ACT tests are designed to monitor lower levels of heparin while others are best at monitoring high levels. When heparin reaches therapeutic maintenance levels, the ACT is usually replaced as a monitoring tool by the PTT test. The ACT test has also been used to monitor the inhibiting effect of a new class of drugs called direct thrombin inhibitors (e.g. bivalirudin) on the clotting system.
How is the sample collected for testing?
A blood sample is taken 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.
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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.