Partial Thromboplastin Time (PTT, aPTT)
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- Also Known As:
- PTT Activated Partial Thromboplastin Time APTT aPTT

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At a Glance
Why Get Tested?
As part of an investigation of a possible bleeding disorder or blood clot (thrombotic episode); to help investigate recurrent miscarriages or diagnose antiphospholipid syndrome (APS); as needed to monitor unfractionated (standard) heparin anticoagulant therapy; as indicated as part of an evaluation before surgery or other invasive procedure
When To Get Tested?
When you have unexplained bleeding, inappropriate blood clotting, or recurrent miscarriages; sometimes when you are on standard heparin anticoagulant therapy; sometimes before a scheduled surgery
Sample Required?
A blood sample drawn by needle from a vein in your arm
Test Preparation Needed?
None; however, a high-fat meal prior to the blood draw may interfere with the test and should be avoided.
You may be able to find your test results on your laboratory’s website or patient portal. However, you are currently at Testing.com. You may have been directed here by your lab’s website in order to provide you with background information about the test(s) you had performed. You will need to return to your lab’s website or portal, or contact your healthcare practitioner in order to obtain your test results.
Testing.com is an award-winning patient education website offering information on laboratory tests. The content on the site, which has been reviewed by laboratory scientists and other medical professionals, provides general explanations of what results might mean for each test listed on the site, such as what a high or low value might suggest to your healthcare practitioner about your health or medical condition.
The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.
If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.
Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called “normal” values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.
While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are “within normal limits.”
For more information, please read the article Reference Ranges and What They Mean.
What is being tested?
The partial thromboplastin time (PTT; also known as activated partial thromboplastin time (aPTT)) 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 sample of blood after substances (reagents) are added. The PTT assesses the amount and the function of certain proteins in the blood called coagulation or clotting 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 stick 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 in a step-by-step process. Through the cascading reactions, threads called fibrin form and crosslink into a net that clings to the injury site and stabilizes it. 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 part of this hemostatic process must function properly and be present in sufficient quantity for normal blood clot formation. If the amount of one or more factors is too low, or if the factors cannot do their job properly, then a stable clot may not form and bleeding continues.
With a PTT, your result is compared to a normal reference interval for clotting time. When your PTT takes longer than normal to clot, the PTT is considered “prolonged.”
When a PTT is used to investigate bleeding or clotting episodes or to rule out a bleeding or clotting disease (e.g., preoperative evaluation), it is often ordered along with a prothrombin time (PT). A health care practitioner will evaluate the results of both tests to help rule out or determine the cause of bleeding or clotting disorder.
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.
- The PTT is used to evaluate the coagulation factors XII, XI, IX, VIII, X, V, II (prothrombin), and I (fibrinogen) as well as prekallikrein (PK) and high molecular weight kininogen (HK).
- A PT test evaluates the coagulation factors VII, X, V, II, and I (fibrinogen).
Common Questions
Health Professionals – LOINC
LOINC Observation Identifiers Names and Codes (LOINC®) is the international standard for identifying health measurements, observations, and documents. It provides a common language to unambiguously identify things you can measure or observe that enables the exchange and aggregation of clinical results for care delivery, outcomes management, and research. Learn More.
Listed in the table below are the LOINC with links to the LOINC detail pages. Please note when you click on the hyperlinked code, you are leaving Testing.com and accessing Loinc.org.
LOINC | LOINC Display Name |
---|---|
16631-4 | aPTT Coagulation 1:1 saline (Bld) [Time] |
3173-2 | aPTT Coag (Bld) [Time] |
49058-1 | aPTT Coag (BldCRRT) [Time] |
43734-3 | aPTT Coagulation 1:1 saline (PPP) [Time] |
14979-9 | aPTT Coag (PPP) [Time] |
View Sources
Sources Used in Current Review
Activated Partial Thromboplastin Time (APTT), Plasma. Mayo Medical Laboratories. Available online at https://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/40935. Accessed March 2019.
(November 23, 2015) Partial Thromboplastin Time, Activated. Medscape. Available online at https://emedicine.medscape.com/article/2085837-overview. Accessed March 2019.
(June 2013) Elevated APTT? How to Best Follow Up. Journal of Family Practice. Available online at https://www.mdedge.com/jfponline/article/76289/cardiology/elevated-aptt-how-best-follow. Accessed March 2019.
Partial Thromboplastin Time (PTT). MedlinePlus. Available online at https://medlineplus.gov/ency/article/003653.htm. Accessed March 2019.
Activated Partial Thromboplastin Time (APTT). University of Rochester Medical Center. Available online at https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=167&contentid=aptt. Accessed March 2019.
Vandiver JW, Vondracek T. Antifactor Xa levels versus activated partial thromboplastin time for monitoring unfractionated heparin. Pharmacotherapy. 2012 Jun;32(6):546-58. Available online at https://www.ncbi.nlm.nih.gov/pubmed/22531940. Accessed March 2019.
Sources Used in Previous Reviews
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(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.
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.
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.
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.
Duke University Medical Center Clinical Coagulation Laboratory Coagulation Test Descriptions [On-line information]. Available online at http://pathology.mc.duke.edu/coag/TestDes.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.
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.
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.
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.
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.
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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.
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(© 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. Accessed September 2010.
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