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Partial thromboplastin time (PTT)
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 bleeding or clotting problems.
APTT; PTT; Activated partial thromboplastin time
How the Test is Performed
The health care provider uses a needle to take blood from one of your veins. The blood collects into an air-tight container. You may be given a bandage to stop any bleeding. If you are taking a medicine called heparin, you will be watched for signs of bleeding.
The lab specialist will add chemicals to the blood sample and see how many seconds it takes for the blood to clot.
How to Prepare for the Test
The health care provider may tell you to stop taking certain medicines before the test. Drugs that can affect the results of a PTT test include antihistamines, heparin, vitamin C (ascorbic acid), and aspirin.
Do not stop taking any medicine without first talking to your doctor.
How the Test will Feel
You may feel slight pain or a sting when the needle is inserted. You may also feel some throbbing at the site after the blood is drawn.
Why the Test is Performed
You may need this test if you have problems with bleeding or blood clotting. When you bleed, a series of actions 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.
In general, clotting should occur between 25 to 35 seconds. If the person is taking blood thinners, clotting takes up to two-and-a-half times longer.
Normal value ranges may vary slightly among different labs. Some lab use different measurements or may test different specimens. Talk to your doctor about your test results.
What Abnormal Results Mean
An abnormal (too long) PTT result may also be due to:
This test is often done on people who may have bleeding problems. The risks of bleeding and hematoma in these patients are a little higher than for people without bleeding problems.
Other slight risks of any blood test can include:
- Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
- Multiple punctures to locate veins
Schafer AI. Approach to the patient with bleeding and thrombosis. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 174.
Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine, Seattle, WA. Also reviewed by A.D.A.M. Health Solutions, Ebix, Inc., Editorial Team: David Zieve, MD, MHA, Bethanne Black, Stephanie Slon, and Nissi Wang.