At a Glance

Why Get Tested?

To monitor treatment with heparin or other blood-thinning medications (anticoagulants) when undergoing heart bypass surgery, coronary angioplasty, or dialysis

When To Get Tested?

When you are receiving high doses of heparin to prevent clotting during and after surgical procedures such as a heart bypass; when heparin levels are too high to allow monitoring with a partial thromboplastin time (PTT) and/or when a rapid result is necessary to monitor treatment

Sample Required?

A blood sample drawn from a vein

Test Preparation Needed?


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 through a vein (intravenously, IV), by injection or continuous infusion. High doses of heparin may be given during medical or surgical 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 anti-factor Xa test. Monitoring is a vital part of the anticoagulation therapy because the blood thinning (anticoagulant) effect of heparin can affect each person a little bit differently. If the amount of heparin administered is not enough to inhibit the body’s clotting system, blood clots may form in blood vessels throughout the body. If there is too much heparin, excessive, even life-threatening, bleeding can occur.

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. This means 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 test used?

The activated clotting time (ACT) is commonly used to monitor treatment with high-dose heparin before, during, and for a short time after medical or surgical procedures that require that blood be prevented from clotting, such as heart bypass surgery, coronary angioplasty, and dialysis.

The ACT is a rapid test that can be performed at the patient’s bedside prior to surgery or other medical procedures. It can also be done in or near the operating room at intervals during and immediately after surgery. (This type of testing is known as point-of-care testing.) ACT testing allows measurement of relatively rapid changes in heparin infusion, helping to achieve and maintain a constant level of anticoagulation throughout the surgical or medical procedure. Once the procedure is complete and the patient has been stabilized, heparin doses are typically decreased.

The ACT measures the inhibiting effect that heparin has on the body’s clotting system, not the actual level of heparin in the blood. 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.

The ACT test is also sometimes used to monitor regular-dose heparin therapy in people with documented lupus anticoagulant (LAC). The PTT test cannot be used in those patients because LAC interferes with the PTT. In rare clinical situations, the ACT test may also be used to monitor the inhibiting effect of a different class of anticoagulation drugs called direct thrombin inhibitors (e.g., argatroban) on the clotting system.

When is it ordered?

The ACT is ordered after an initial dose (bolus) of heparin and before the start of an open heart surgery or other procedure that requires a high level of anticoagulation. During surgery, the ACT is measured at intervals to achieve and maintain a steady level of heparin anticoagulation. After surgery, the ACT is monitored until the person has stabilized and the heparin dosage has been reduced and/or neutralized with a counter agent such as protamine sulfate.

Occasionally, the ACT may be measured during a bleeding episode or used as part of a bedside evaluation of a person’s heparin anticoagulation level, particularly if the person has lupus anticoagulant (LAC).

It may also be used when someone is receiving direct thrombin inhibitor therapy (e.g., argatroban).

What does the test result mean?

The ACT is measured in seconds: the longer the time to clot, the higher the degree of clotting inhibition (anticoagulation). During surgery, the ACT is kept above a lower time limit, a limit at which most people will not form blood clots. There is no widespread agreement of exactly what this lower limit should be. It will vary from hospital to hospital and depends to some degree on the method used to determine ACT.

It is important to evaluate how the person is responding to this ACT lower limit and to the amount of heparin that person is being given. The amount of heparin needed to reach and maintain a certain ACT (for instance, 300 seconds) will vary as will the body’s clotting potential at that ACT. If there are clotting or bleeding problems, the dosages and ACT may need to be adjusted accordingly. After surgery, the ACT may be maintained within a narrow range (for instance, 175-225 seconds) until the person has stabilized.

Is ACT ever done in the central laboratory?

The ACT test is rarely performed in the central laboratory. It is a point-of-care test that must be performed immediately after the blood is collected, close to the patient, usually at the bedside, in the operating room, or in a satellite laboratory close to these locations. The ACT result is needed quickly to guide treatment.

Can lupus anticoagulant interfere with the ACT test?

In some patients, the presence of a lupus anticoagulant (LAC) has been shown to prolong the ACT, but in other cases the ACT may be relatively unaffected. Nevertheless, the presence of LAC has been shown to interfere with certain ACT testing. Therefore, it is important for the laboratory to follow the manufacturer’s instructions and determine if the test is suitable to monitor heparin therapy in a person with documented history of LAC or antiphospholipid syndrome.

Is there anything else I should know?

The ACT and PTT results are not interchangeable. In the area where they overlap (upper measurements of PTT and lower levels of ACT), they have poor correlation. ACT and PTT results should be evaluated independently. It is better to determine someone’s heparin anticoagulant requirements, stabilize the person, and then change the monitoring tool.

The ACT may be influenced by a person’s platelet count and platelet function. Platelets that are activated during surgery often become dysfunctional, and both surgery and heparin can sometimes cause platelet numbers to decrease (thrombocytopenia).

The temperature of the blood may also affect ACT results; the blood tends to cool during surgery as it is mechanically filtered and oxygenated. Acquired and inherited conditions such as coagulation factor deficiencies and in patients receiving oral anticoagulants or with liver disease may also affect ACT results.

With high doses of heparin and in individuals with a prolonged PTT prior to heparin anticoagulation (e.g., lupus anticoagulant), the PTT cannot be used to monitor heparin therapy. In such situations, the ACT and Heparin Anti-Xa tests are used instead of the PTT, respectively.

Direct Thrombin Inhibitors (e.g., argatroban) will prolong the ACT.


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