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Prothrombin time (PT)

Definition

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

A related blood test is partial thromboplastin time (PTT).

Alternative Names

PT; Pro-time; Anticoagulant-prothrombin time; Clotting time: protime; INR; International normalized ratio

How the Test is Performed

A blood sample is needed. If you are taking ant blood-thinning medicines, you will be watched for signs of bleeding.

How to Prepare for the Test

Certain medicines can change blood test results.

How the Test will Feel

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or slight bruising. This soon goes away.

Why the Test is Performed

The most common reason to perform this test is to monitor your levels when you are taking a blood-thinning medicine called warfarin. You are likely taking this medicine to prevent blood clots.

Your provider will check your PT regularly.

You may also need this test to:

Normal Results

PT is measured in seconds. Most of the time, results are given as what is called INR (international normalized ratio).

If you are not taking blood thinning medicines, such as warfarin, the normal range for your PT results is:

If you are taking warfarin to prevent blood clots, your doctor will most likely choose to keep your INR between 2.0 and 3.0.

Ask your doctor what result is right for you.

Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results

What Abnormal Results Mean

If you are not taking blood thinning medicines, such as warfarin, an INR result above 1.1 means your blood is clotting more slowly than normal. This may be due to:

If you are taking warfarin to prevent clots your doctor will most likely choose to keep your INR between 2.0 and 3.0:

A PT result that is too high or too low in someone who is taking warfarin (Coumadin) may be due to:

Your provider will teach you about taking warfarin (Coumadin) the proper way.

Risks

This test is often done on people who may have bleeding problems. Their risk of bleeding is slightly higher than for people without bleeding problems.

Other slight risks can include:

References

Fink LM, Marlar RA, Miller JL. Antithrombotic therapy. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 42.

Schmaier AH, Miller JL. Coagulation and fibrinolysis. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 39.


Review Date: 1/27/2015
Reviewed By: Yi-Bin Chen, MD, Leukemia/Bone Marrow Transplant Program, Massachusetts General Hospital, Boston, MA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.
The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997- A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.
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