Test Name: Clotting time - CT
Normal Range: -
To monitor heparin and other anticoagulation when undergoing cardiopulmonary bypass, coronary angioplasty, and dialysis
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 APTT test and/or when a rapid result is necessary to monitor treatment
A blood sample taken from a vein in your arm
Activated coagulation time (ACT) is a blood clotting test that is used primarily to monitor high doses of heparin anticoagulant therapy. Heparin is a blood thinner (anticoagulant) 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 activated partial thromboplastin time (APTT) 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. When blood is in contact with artificial surfaces this 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 blood clotting and bleeding. At this level of anticoagulation, the APTT is no longer clinically useful as a monitoring tool. The APTT test involves an in vitro (test-tube) clotting reaction and at high levels of heparin the blood 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 APTT, it measures the inhibiting effect that heparin and other antithrombotic medications have on the body’s clotting system, and not the actual level of heparin in the blood. ACT testing allows relatively rapid changes in the dose of 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 the therapeutic maintenance levels, the ACT is usually replaced as a monitoring tool by the APTT 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.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.