Definition:
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.
Alternative Names:
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 laboratory 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 drugs before the test. Drugs that can affect the results of a PTT test include antihistamines, vitamin C (ascorbic acid), aspirin, and chlorpromazine (Thorazine). Do not stop taking any medicine without first talking to your doctor.
How the test will feel:
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed:
Your doctor may order this test if you have problems with bleeding or blood clotting. When you bleed, the body launches a series of activities that help the blood clot. This is called the coagulation cascade. There are three pathways to this event. The PTT test looks at special proteins, called factors, found in two of these pathways. 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.
References:
Schafer AI. Approach to the patient with bleeding and thrombosis. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 178.
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