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Also known as: Activated Coagulation Time
Formal name: Activated Clotting Time

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

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?


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.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

Pagana, K. D. & Pagana, T. J. (© 2011). Mosby’s Diagnostic and Laboratory Test Reference 10th Edition: Mosby, Inc., Saint Louis, MO. Pp 10-12.

Pagnani, G. et. al. (Updated 2009 May 29). Massachusetts General Hospital Activated Clotting Time performed on the Medtronic ACT PlusTM. Massachusetts General Hospital. [On-line information]. PDF available for download at through Accessed July 2011.

Calkins et al. (2007 June) Catheter and Surgical Ablation of AF. Society of Thoracic Surgeons, Heart Rhythm, Vol 4, No 6 [On-line information]. PDF available for download at through Accessed July 2011.

Marmur, J. et al. (2010 March 8). Avoiding Intelligence Failures in the Cardiac Catheterization Laboratory: Strategies for the Safe and Rational Use of Dalteparin or Enoxaparin during Percutaneous Coronary Intervention. Medscape Today from J Invasive Cardiol. 2009;21(12):653-64 [On-line information]. Available online at through Accessed July 2011.

Ganter M, Hofer C. Coagulation Monitoring: Current Techniques and Clinical Use of Viscoelastic Point-of-Care Coagulation Devices. Anesthesia & Analgesia May 2008 vol. 106 no. 5, Pp 1366-1375. Available online at through Accessed August 2011.

Sources Used in Previous Reviews

Pagana, Kathleen D. & Pagana, Timothy J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 9-11.

Clarke, W. and Dufour, D. R., Editors (2006). Contemporary Practice in Clinical Chemistry, AACC Press, Washington, DC. Pg 233.

Wu, A. (2006). Tietz Clinical Guide to Laboratory Tests, Fourth Edition. Saunders Elsevier, St. Louis, Missouri. Pp 44-46.

Ganter, M. et. al. (2005). Monitoring Activated Clotting Time for Combined Heparin and Aprotinin Application: An In Vitro Evaluation of a New Aprotinin-Insensitive Test Using SONOCLOT. Anesth Analg 2005; 101:308-314. [On-line journal article]. Available online through Accessed on 8/18/07.

(2007 May 29). Activated Clotting Time. Massachusetts General Hospital Pathology Service [On-line information]. Available online through Accessed on 8/12/07.

Baird, C. et. al. (2007). Anticoagulation and Pediatric Extracorporeal Membrane Oxygenation: Impact of Activated Clotting Time and Heparin Dose on Survival. Ann Thorac Surg 2007;83:912-920. [On-line journal article]. Available online through Accessed on 8/18/07.

Jabr K, Johnson JH, McDonald MH, Walsh DL, Martin WD, Johnson AC, Pickett JM, Shantha-Martin U. Plasma-modified ACT can be used to monitor bivalirudin (Angiomax) anticoagulation for on-pump cardiopulmonary bypass surgery in a patient with heparin-induced thrombocytopenia. J Extra Corpor Technol. 2004; 36:174-7.

Tremey B, Szekely B, Schlumberger S, François D, Liu N, Sievert K, Fischler M. Anticoagulation monitoring during vascular surgery: accuracy of the Hemochron low range activated clotting time (ACT-LR). Br J Anaesth. 2006; 97:453-9.

Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.

(2002 November 19, Updated). Activated Clotting Time. Mass. Gen Hospital, Pathology Service, Laboratory Medicine Coag Test Handbook Index [On-line information]. Available online through

Carville, D. and Guyer, K. (2000 September). Hemostasis testing: Past, present, and future. Medical devicelink [On-line information]. Available online through

(1998 Spring). Anticoagulation Services Newsletter. Detroit Medical Center Department of Pharmacy Services, v 3 (2) [On-line Newsletter]. PDF available for download at

Chapter 2: Mediastinal Bleeding / Emergency Sternotomy, Manipulations on Cardiopulmonary Bypass. Stony Brook State Univ of NY, Dept of Surgery, Cardiac Surgical Residents Handbook.

Zwischenberger, J. and Bartlett, R. What is ECLS? An Introduction to Extracorporeal Life Support. University of Michigan Health System [On-line information]. Available online through

Menta, S. (1999 Spring). The Coagulation Cascade. Physiology Disorders Evaluation, College of Medicine, Univ of Florida.

Elstrom, R. (2001 November 25, Updated). PTT. MEDLINEplus Health Information, Medical Encyclopedia [On-line information]. Available online at