Also Known As
Activated Coagulation Time
Formal Name
Activated Clotting Time
This article was last reviewed on
This article waslast modified on 19 July 2023.
At a Glance
Why Get Tested?

To monitor heparin and other anticoagulation when undergoing cardiopulmonary bypass, coronary angioplasty, and dialysis

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 APTT test and/or when a rapid result is necessary to monitor treatment

Sample Required?

A blood sample taken from a vein in your arm

Test Preparation Needed?


Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

What is being tested?

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 anticoagulation therapy because a particular quantity of heparin will affect each person a slightly 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 result 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. This is referred to as a Point of Care Test (POCT). 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. 

Accordion Title
Common Questions
  • How is it used?

    The ACT test is used to monitor the effect of high dose heparin before, during, and shortly after surgery requiring intense anticoagulation measures, such as cardiac bypass surgery and cardiac angioplasty. It is requested in situations where the APTT test is not clinically useful (e.g. with very high doses of heparin) or the APTT laboratory result takes too long. 

  • When is it requested?

    The ACT is requested after someone has received 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 patient has stabilised and the heparin dosage has been reduced and/or neutralised with a counter agent.

    Occasionally, the ACT may be measured during a bleeding episode or used as part of a bedside evaluation of a patient’s heparin anticoagulation level. This is particularly if they have the "lupus anticoagulant", which can interfere with the more usual activated partial thromboplastin (APTT) test.

    It may be used in patients receiving direct thrombin inhibitor therapy (e.g. bivalirudin).

  • What does the test result mean?

    The ACT is measured in seconds: the higher it is the higher the degree of clotting inhibition i.e. the thinner the blood. During surgery, the ACT is maintained above a lower limit, a limit at which most people will not form blood clots. It is important to evaluate how the patient is responding to this ACT lower limit and to the amount of heparin they are receiving. The amount of heparin needed to achieve 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 heparin dosage and ACT may be adjusted accordingly. After surgery, the ACT may be maintained within a narrow range (for instance, 175 – 225 seconds) until the patient has stabilised.

  • Is there anything else I should know?

    ACT and APTT results are not interchangeable. In the area where they overlap (upper measurements of APTT and lower levels of ACT), they have poor correlation. ACT and APTT results should be evaluated independently. It is better to determine a patient’s heparin anticoagulant requirements, stabilise them, and then change the test used for anticoagulation monitoring.

    The ACT may be influenced by a person's platelet count and platelet function. Both surgery and heparin can cause thrombocytopenia (low platelet count) resulting in a prolonged ACT. Medicines such as aspirin cause platelet dysfunction resulting in a prolonged ACT.  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 may also affect ACT results.

  • Is ACT ever done in the laboratory?

    Rarely is the test performed in a main hospital or central laboratory.  The sample is not stable so the test must be done immediately, close to the patient, usually at the patient's bedside, in theatre or other point of care testing (POCT) system.

  • Can “lupus anticoagulant” interfere with the ACT test?

    The ACT test may be affected by lupus anticoagulant, although this is not always the case. The test should therefore be interpreted with caution, in conjunction with the manufacturer’s instructions and on a case-by-case basis.