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).
How the Test is Performed
A blood sample is needed. If you are taking any blood-thinning medicines, you will be watched for signs of bleeding.
How the Test will Feel
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.
How to Prepare for the Test
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.
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.
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.
Other slight risks may include:
- Fainting or feeling lightheaded
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
- Multiple punctures to locate veins
What Abnormal Results Mean
An abnormal (too long) PTT result may also be due to:
- Bleeding disorders, a group of conditions in which there is a problem with the body's blood clotting process
- Disorder in which the proteins that control blood clotting become over active (disseminated intravascular coagulation)
- Liver disease
- Difficulty absorbing nutrients from food (malabsorption)
- Low level of vitamin K
Why the Test is Performed
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 help 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.
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.
Ortel TL. Antithrombotic therapy. McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 23rd ed. St Louis, MO: Elsevier; 2017:chap 42.