The Test Sample
What is being tested?
ACT is a blood clotting test that is used primarily to monitor high doses of unfractionated (standard) heparin anticoagulant therapy. Heparin is a blood clotting inhibitor that is usually given either intravenously (IV) or by injection. 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) test or the heparin antifactor Xa test. Monitoring is a vital part of the anticoagulation therapy because a particular quantity of heparin will 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 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.
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 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 stabilized, heparin doses are typically 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 of heparin. When heparin reaches therapeutic maintenance levels, the ACT is usually replaced as a monitoring tool by the PTT test.
The ACT test is sometimes used to monitor regular-dose heparin therapy in people with documented lupus anticoagulant (LAC). The PTT test cannot be used in those patients becuase LAC interferes with the PTT test.
The ACT test may also be used to monitor the inhibiting effect of a new class of drugs called direct thrombin inhibitors (e.g., bivalirudin, argatroban) on the clotting system.
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.