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.

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.

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.

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.

Your doctor may order this test if you have problems with bleeding or blood clotting. 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.

The normal value will vary between laboratories. 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.

An abnormal (too long) PTT result may be due to:

• Cirrhosis
• Disseminated intravascular coagulation (DIC)
• Factor XII deficiency
• Hemophilia A
• Hemophilia B
• Hypofibrinogenemia
• Malabsorption
• Von Willebrand's disease
• Lupus anticoagulants

This test is often done on people who may have bleeding problems. The risks of bleeding and hematoma in these patients are slightly greater than for people without bleeding problems.

In general, risks of any blood test may 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

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.

Ferri FF. Ferri’s Clinical Advisor: Instant Diagnosis and Treatment. 2005 ed. St. Louis, Mo: Mosby; 2005:1365.

Hoffman R, Benz EJ, Shattil SS, et al. Hematology: Basic Principles and Practice. 4th ed. Orlando, Fl: Churchill Livingstone; 2005:2004-2005.