At a Glance
Why Get Tested?
When to Get Tested?
When you have unexplained bleeding or blood clotting; when you are on unfractionated (standard) heparin anticoagulant therapy; sometimes as part of a pre-surgical screen
A blood sample drawn by needle from a vein in the arm
Test Preparation Needed?
The Test Sample
What is being tested?
The partial thromboplastin time (PTT) is a screening test that helps evaluate a person's ability to form blood clots appropriately by measuring the time it takes (in seconds) for a clot to form in a test tube when specific substances (reagents) are added to a sample of plasma. By measuring the time it takes to form the clot, the PTT assesses the amount as well as the function of certain coagulation factors that are part of hemostasis.
When body tissue(s) or blood vessel walls are injured, bleeding occurs and a process called hemostasis is initiated. Small, sticky cell fragments called platelets adhere to and then aggregate at the injury site. This begins the activation of the coagulation cascade system, the components of which are known as coagulation factors. Through the activation of a series of other factors, fibrinogen (Factor I) is converted to fibrin, a thread-like material that crosslinks together to form a fibrin net that adheres to the injury site. This fibrin net, along with platelets, produces a stable blood clot. Blood clots are formed by this process to seal off injuries to blood vessels, to prevent further blood loss, and to give the damaged areas time to heal.
Each component of the 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. Excessive bleeding, whether external or internal, will lead to serious and possibly life-threatening episodes.
With a PTT, a person's sample is compared to a normal reference interval. When a person's PTT takes longer than normal to clot, the PTT is said to be "prolonged." A prolonged PTT may be due to a condition that causes a decrease in the amount of one or more coagulation factors, inhibition by certain antibodies, or dysfunction of one or more coagulation factors. When used to investigate bleeding or clotting episodes, a PTT is often ordered in conjunction with a prothrombin time (PT) test. A doctor will evaluate the results of the two tests together 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 specifically evaluates the coagulation factors that are often referred to as the intrinsic coagulation and common pathways while the PT evaluates those coagulation factors that are part of the extrinsic and common pathways. (For more on this, see the explanation of 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 cause interference with the test and should be avoided.
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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. (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.
Jacobs DS, DeMott WR, Oxley DK. Laboratory Test Handbook, 5th ed.Lexicomp: Hudson, OH. Coagulation: EM Van Cott & M Laposata, Pp 327-358, 2001.
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.
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://18.104.22.168/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.
Activated Partial Thromboplastin Time. Florida Hospital Cancer Institute, Clinical and Research Laboratories Coagulation Test Panels [On-line information]. Available online at http://www.fhci-labs.com/researchlabs/clinicallabs/hemostasisandthrombosis/panels.htm through http://www.fhci-labs.com.
PTT-LA. Florida Hospital Cancer Institute, Clinical and Research Laboratories Coagulation Test Panels [On-line information]. Available online at http://www.fhci-labs.com/researchlabs/clinicallabs/hemostasisandthrombosis/panels.htm through http://www.fhci-labs.com/.
Duke University Medical Center Clinical Coagulation Laboratory Coagulation Test Descriptions [On-line information]. Available online at http://pathology.mc.duke.edu/coag/TestDes.htm through http://pathology.mc.duke.edu.
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.
Coagulation Test Panels. Clinical and Research Laboratories, Florida Hospital Cancer Institute [On-line information]. Available online at http://www.fhci-labs.com/researchlabs/clinicallabs/hemostasisandthrombosis/panels.htm through http://www.fhci-labs.com.
Menta, S. (1999 Spring). The Coagulation Cascade. Physiology Disorders Evaluation, College of Medicine, Univ of Florida [On-line information]. Available online at http://www.medinfo.ufl.edu/year2/coag/title.html through http://www.medinfo.ufl.edu.
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.
What is a Bleeding Disorder? National Hemophilia Foundation, Bleeding Disorders Info Center [On-line information]. Available online at http://www.hemophilia.org/bdi/bdi_general.htm through http://www.hemophilia.org.
DeLoughery, T. (1999 March 15). Tests of Hemostasis and Thrombosis. OHSU [Online student handout]. Available online at http://www.ohsu.edu/som-hemonc/handouts/deloughery/printtest.html through http://www.ohsu.edu.
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.