What is being tested?ACT is a 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 injection. In moderate doses it is used to help prevent and treat
thromboemboli. At concentrations achieved with moderate doses, heparin 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 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 clotting reaction and at high levels of heparin it will not clot.
the ACT is a rapid test that can be performed at the patient’s bedside prior to surgery or other medical procedures and in or near the operating room at intervals during 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 stabilized, 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 of heparin. When heparin reaches therapeutic maintenance levels, the ACT is usually replaced as a monitoring tool by the PTT test. The ACT has also been used to monitor the inhibiting effect of a new class of drugs called direct thrombin inhibitors on the clotting system. One example is the drug bivalirudin.
