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.
How to prepare for the test
Make sure your doctor knows about all the medicines you are taking, including over-the-counter medicines, herbs, and supplements. 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.
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, you may feel some throbbing.
Why the test is performed
Your doctor may order this test if you have signs of a blood clotting disorder.
If you are taking a blood-thinning medicine called warfarin, your health care provider will check your PT, or prothrombin time, regularly.
When you bleed, the body launches a series of activities that help the blood clot. This is called the coagulation cascade. The PT test looks at special proteins (called coagulation factors) that are involved in this event, and measures their ability to help blood clot.
The normal range for someone who is not taking a blood thinner medication is 11 - 13.5 seconds.
The PT result will be longer in persons who take blood thinners. Ask your doctor what result is right for you.
Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
The examples above show the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.
What abnormal results mean
When any of the blood clotting factors are lacking or not working properly, the PT is prolonged.
A PT result that is too high or too low in someone who is taking warfarin (Coumadin) may be due to:
The wrong dose of medication
Taking certain over-the-counter medicines, vitamins, supplements, cold medicines, antibiotics, or other drugs
Eating a food that changes the way the blood-thinning medication works in your body
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:
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, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 178.
Schmaier AH. Laboratory evaluation of hemostatic and thrombotic disorders. In: Hoffman R, Benz EJ Jr, Shattil SJ, et al, eds. Hoffman Hematology: Basic Principles and Practice. 5th ed. Philadelphia, Pa: Churchill Livingstone Elsevier; 2008:chap 122.
David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M. Health Solutions, Ebix, Inc.