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PTT

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Also known as: Activated Partial Thromboplastin Time; aPTT; APTT
Formal name: Partial Thromboplastin Time

At a Glance

Why Get Tested?

As part of an investigation of a possible bleeding disorder or blood clot (thrombotic episode); to monitor unfractionated (standard) heparin anticoagulant therapy; as part of screening before surgery or other invasive procedure

When to Get Tested?

When you have unexplained bleeding, inappropriate blood cloting, or recurrent miscarriages; when you are on standard heparin anticoagulant therapy; sometimes as part of a pre-surgical screen

Sample Required?

A blood sample drawn by needle from a vein in your arm

Test Preparation Needed?

None

The Test Sample

What is being tested?

The partial thromboplastin time (PTT) is a screening test that helps evaluate a person's ability to appropriately form blood clots. It measures the number of seconds it takes for a clot to form in a person's sample of blood after substances (reagents) are added. The PTT assesses the amount and the function of certain proteins called coagulation factors that are an important part of blood clot formation.

When body tissue(s) or blood vessel walls are injured, bleeding occurs and a process called hemostasis begins. Small cell fragments called platelets adhere to and then clump (aggregate) at the injury site. At the same time, a process called the coagulation cascade begins and coagulation factors are activated. Through the cascading reactions, threads called fibrin form and crosslink into a net that adheres to the injury site and stabilizes it. Along with the platelets adhering, this forms a stable blood clot to seal off injuries to blood vessels, prevents additional blood loss, and gives the damaged areas time to heal.

Each component of this hemostatic process must function properly and be present in sufficient quantity for normal blood clot formation. If there is a deficiency in one or more of these factors, or if the factors function abnormally, then a stable clot may not form and bleeding continues.

With a PTT, a person's sample is compared to a normal reference interval for clotting time. When a person's PTT takes longer than normal to clot, the PTT is considered "prolonged." A prolonged PTT may be due to a condition that decreases or creates a dysfunction in one or more coagulation factors. Less often, it may be due to a condition in which the body produces certain antibodies directed against one or more coagulation factors, affecting their function.

Sometimes a PTT may be prolonged because the person tested produces an autoantibody called an antiphospholipid antibody that interferes with the test. This type of antibody affects the results of the test because it targets substances called phospholipids that are used in the PTT. Though antiphospholipid antibodies can prolong the PTT test result, in the body they are associated with excessive clotting. A person who produces these antibodies may be at an increased risk for a blood clot. A PTT maybe used as part of an evaluation of a person with signs and symptoms of excessive clotting or antiphospholipid syndrome. (See the article on Antiphospholipid Antibodies for additional details.)

When a PTT is used to investigate bleeding or clotting episodes, it is often ordered along with a prothrombin time (PT). A health practitioner will evaluate the results of both tests to help determine the cause of bleeding or clotting episode(s).

It is now understood that coagulation tests such as the PT and PTT are based on what happens artificially in the test setting (in vitro) and thus do not necessarily reflect what actually happens in the body (in vivo). Nevertheless, they can be used to evaluate certain components of the hemostasis system. The PTT and PT tests each evaluate coagulation factors that are part of different groups of chemical reaction pathways in the cascade, called the intrinsic, extrinsic, and common pathways. (For more on this, see the article on the Coagulation Cascade.)

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; however, a high-fat meal prior to the blood draw may interfere with the test and should be avoided.

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

Dugdale, D.C. (Updated 2011 February 13) Partial Thromboplastin Time (PTT). National Institutes of Health Medline Plus. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003653.htm through http://www.nlm.nih.gov/. Accessed May 2014.

Hammami, M. B. (Updated 2013 February 20). Partial Thromboplastin Time, Activated. Medscape. Available online at http://emedicine.medscape.com/article/2085837-overview through http://emedicine.medscape.com. Accessed May 2014.

(© 1995–2014). Activated Partial Thromboplastin Time (APTT), Plasma. Mayo Medical Laboratories. Available online at http://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/9058 through http://www.mayomedicallaboratories.com. Accessed May 2014.

Tuazon, S.A. et al. (Updated 2012 September 4). Prothrombin Time. Available online at http://emedicine.medscape.com/article/2086058-overview through http://emedicine.medscape.com. Accessed May 2014.

Pollak, E.S. (Updated 2012 April 6). von Willebrand Disease. Medscape. Available online at http://emedicine.medscape.com/article/206996-overview through http://emedicine.medscape.com. Accessed May 2014.

Pagana, K. D. & Pagana, T. J. (© 2014). Mosby's Manual of Diagnostic and Laboratory Tests Reference 5th Edition: Mosby, Inc., Saint Louis, MO. Pp 383-386.

(Reviewed 2013 February). Recurrent Miscarriage Evaluation/Coagulation Panel. Quest Diagnostics. Available online at http://www.questdiagnostics.com/testcenter/testguide.action?dc=TS_Recurrent_Miscarriage through http://www.questdiagnostics.com. Accessed May 2014.

Moake, J. (Reviewed 2012 July). Excessive Bleeding. The Merck Manual. Available online at http://www.merckmanuals.com/professional/hematology_and_oncology/hemostasis/excessive_bleeding.html?qt=partial%20thromboplastin&alt=sh through http://www.merckmanuals.com. Accessed May 2014.

Moake, J. (Reviewed 2013 December). Coagulation Disorders Caused by Circulating Anticoagulants. The Merck Manual. Available online through http://www.merckmanuals.com. Accessed May 2014.

Eke, Sancar. (Reviewed 2012 March). Medscape. Heparin-Induced Thrombocytopenia Treatment & Management. Available online at http://emedicine.medscape.com/article/1357846-treatment through http://emedicine.medscape.com. Accessed May 2014.

Szigeti, Reka. (Updated 2012 October). Anti-Xa Assay (Heparin Assay). Medscape. Available online at http://emedicine.medscape.com/article/2085000-overview#a30 through http://emedicine.medscape.com. Accessed May 2014.

Sources Used in Previous Reviews

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, Modified). Activated Partial Thromboplastin Time. Mass Gen. Hospital Pathology Service Laboratory Medicine [On-line information, Coag Test Handbook]. Available online at http://www.mgh.harvard.edu/labmed/lab/coag/handbook/co003400.htm#co003400 through http://www.mgh.harvard.edu.

Activated Partial Thromboplastin Time. MCL Web Resources, Topics in Hemostasis [On-line information]. Available online at http://155.58.145.40/hemostasis/PTT1.html.

Olson, J. (1999 September). Addressing clinical etiologies of a prolonged aPTT. CAP Today, In the News [On-line Newsletter]. Available online at http://www.cap.org/captoday/CaseStudy/coag4.html through http://www.cap.org.

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Elstrom, R. (2001 November 25, Updated ). PTT. MEDLINEplus Health Information, Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003653.htm.

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Elstrom, R. (2001 November 25, Updated). PT. MEDLINEplus Health Information, Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003652.htm.

Elstrom, R. (2001 November 25, Updated). PTT. MEDLINEplus Health Information, Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003653.htm.

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Pagana, Kathleen D. & Pagana, Timothy J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 705-707.

Clarke, W. and Dufour, D. R., Editors (2006). Contemporary Practice in Clinical Chemistry, AACC Press, Washington, DC. Harris, N. et. al. Chapter 19: Assessment of Hemostasis in the Clinical Laboratory. Pp 227-239.

Dugdale, D. (Updated 2010 August 19). Partial thromboplastin time (PTT). MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003653.htm. Accessed September 2010.

(© 1995–2010). Unit Code 9058: Activated Partial Thromboplastin Time (APTT), Plasma. Mayo Clinic, Mayo Medical Laboratories [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Overview/9058 through http://www.mayomedicallaboratories.com. Accessed September 2010.

Ballas, M. and Kraut, E. (2008 April 15). Bleeding and Bruising: A Diagnostic Work-up. Am Fam Physician. 2008 Apr 15;77(8):1117-1124. [On-line information]. Available online at http://www.aafp.org/afp/2008/0415/p1117.html through http://www.aafp.org. Accessed September 2010.

(© 1996 – 2010). 3895: Activated Partial Thromboplastin Time (APTT). Specialty Laboratories [On-line information]. Available online at http://www.specialtylabs.com/tests/details.asp?id=3895 through http://www.specialtylabs.com. September June 2010.

(© 1995-2010). Blood Test: Partial Thromboplastin Time (PTT). KidsHealth from Nemours [On-line information]. Available online at http://kidshealth.org/parent/system/medical/test_ptt.html through http://kidshealth.org. Accessed September 2010.

Moake, J. (Revised 2009 June). Uncommon Hereditary Coagulation Disorders. Merck Manual for Healthcare Professionals [On-line information]. Available online at http://www.merck.com/mmpe/sec11/ch136/ch136e.html through http://www.merck.com. Accessed September 2010.

Pagana, K. D. & Pagana, T. J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 705-707.

Wu, A. (© 2006). Tietz Clinical Guide to Laboratory Tests, 4th Edition: Saunders Elsevier, St. Louis, MO. Pp 46-47, 932-933.

(January 2008) How to Interpret and Pursue an Abnormal Prothrombin Time, Activated Partial Thromboplastin Time and Bleeding Time. MyoMedialLaboratories.com, Vol. 33 No.1. PDF available for download at http://www.mayomedicallaboratories.com/mediax/articles/communique/2008/mc2831-0108.pdf through http://www.mayomedicallaboratories.com. Accessed September 2010.

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Florida Hospital, Center for Thrombosis Research, © 2008, PTT-LA. Available online at http://www.fhthrombosis.com/PTT-LA through http://www.fhthrombosis.com. Accessed October 2010.

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