Prothrombin time (PT) is a blood test that measures the time it takes for the liquid portion (plasma) of your blood to clot.

See also: Partial thromboplastin time (PTT)

The health care provider uses a needle to take blood from one of your veins. The blood collects into an airtight 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 long it takes for the plasma to clot.

Your doctor may tell you to stop taking certain drugs before the test. For example, blood thinners can affect the results of this test.

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.

Veins and arteries vary in size from one patient 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.

The PT is a broad screening test for many types of bleeding disorders. Your doctor may order this test if you have signs of a blood clotting disorder.

The normal range is 11 to 13.5 seconds. However, "normal" varies from lab to lab.

The PT time will be longer in persons who take blood thinners.

Increased PT times may be due to:

• Bile duct obstruction
• Cirrhosis
• Disseminated intravascular coagulation
• Hepatitis
• Malabsorption
• Vitamin K deficiency
• Coumadin (warfarin) therapy
• Factor VII deficiency
• Factor X deficiency
• Factor II (prothrombin) deficiency
• Factor V deficiency
• Factor I (fibrinogen) deficiency

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 PT test looks at special proteins, called factors, found in one of these pathways. It measures the clotting ability of factors I (fibrinogen), II (prothrombin), V, VII, and X. When any of these factors are lacking, the PT is prolonged.

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.