Partial thromboplastin time (PTT) is a blood test that looks at how long it takes for blood to clot. It can help tell if you have a bleeding problem or if your blood does not clot properly.
A related blood test is prothrombin time (PT).
APTT; PTT; Activated partial thromboplastin time
A blood sample is needed. If you are taking any blood-thinning medicines, you will be watched for signs of bleeding.
Your health care provider may tell you to temporarily stop taking medicines that may affect the test results. Be sure to tell your provider about all the medicines you take. Also, tell your provider about any herbal remedies you take.
When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or slight bruising. This soon goes away.
You may need this test if you have problems with bleeding or your blood does not clot properly. When you bleed, a series of actions involving many different proteins (clotting factors) take place in the body that helps the blood clot. This is called the coagulation cascade. The PTT test looks at some of the proteins or factors involved in this process and measures their ability to help blood clot.
The test may also be used to monitor patients who are taking heparin, a blood thinner.
A PTT test is usually done with other tests, such as the prothrombin test.
In general, clotting should occur within 25 to 35 seconds. If the person is taking blood thinners, clotting takes up to 2 ½ times longer.
Normal value ranges may vary slightly among different labs. Some labs use different measurements or may test different specimens. Talk to your provider about the meaning of your specific test results.
An abnormal (too long) PTT result may also be due to:
There is little risk involved with having your blood taken. Veins and arteries vary in size from one person to another, and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight, but may include:
This test is often done on people who may have bleeding problems. Their risk of bleeding is slightly higher than it is for people without bleeding problems.
Chernecky CC, Berger BJ. Activated partial thromboplastin substitution test - diagnostic. In: Chernecky CC, Berger BJ, eds. Laboratory Tests and Diagnostic Procedures. 6th ed. St Louis, MO: Elsevier Saunders; 2013:101-103.
Lee GM, Ortel TL. Antithrombotic therapy. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 43.
Schafer AI. Approach to the patient with bleeding and thrombosis. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 162.
Reviewed By: Todd Gersten, MD, Hematology/Oncology, Florida Cancer Specialists & Research Institute, Wellington, FL. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.