Prothrombin Time Test

Written by Heather Ross | Published on July 25, 2012
Medically Reviewed by Brenda B. Spriggs, MD, MPH, FACP


A prothrombin time (PT) test, measures the amount of time it takes for your blood plasma to clot. Prothrombin, also known as factor ll, is just one of many plasma proteins involved in the clotting process.

Why Is a Prothrombin Time Test Performed?

When you get a cut and your blood vessel is ruptured, blood platelets collect at the site of the wound, creating a temporary plug to stop the bleeding. In order to create a strong blood clot, a series of 12 plasma proteins, or coagulation “factors,” act together to create a substance called fibrin that seals the wound.

If your body does not create a certain coagulation factor or does so incorrectly, it can be due to a bleeding disorder known as hemophilia. Symptoms of a bleeding disorder include:

  • easy bruising
  • bleeding that won’t stop even after applying pressure to the wound
  • heavy menstrual periods
  • blood in the urine
  • swollen or painful joints
  • nosebleeds

If your doctor suspects that you may have a bleeding disorder, he or she may order a PT test to help make the diagnosis. Your doctor may also order a PT test to make sure that your blood is clotting normally before you undergo major surgery, even if you have no symptoms of a bleeding disorder.

If you are taking the blood-thinning medication warfarin, your doctor will order regular PT tests to ensure that that you are not taking so much medication that you are susceptible to excessive bleeding.

The test may also be used to check how the blood of people with liver disease or vitamin K deficiency clots. This is because these conditions can cause a bleeding disorder to develop.

How Is a Prothrombin Time Test Performed?

Tell your doctor about all medications and supplements you are taking, and he or she will advise you whether or not to stop taking them before the test. The doctor may ask you to stop taking your blood-thinning medication because it can affect the results of the test. You will not need to fast beforehand.

You will need to have your blood drawn in order to have a PT test. Having blood drawn is an outpatient procedure, usually performed at a diagnostic lab. The procedure takes only a few minutes and is relatively painless.

A nurse will use a small needle to withdraw blood from a vein (usually in your arm or hand), and a laboratory specialist will add chemicals to the blood to see how long it takes for a clot to form.

What Risks Are Associated with a Prothrombin Time Test?

There are very few risks associated with having your blood drawn for a PT test. However, because the test is often performed on patients who have bleeding disorders, they are at a slightly higher risk for excessive bleeding and hematoma (blood that accumulates under the skin).

You may feel slightly faint or feel some soreness or pain at the site where your blood was drawn. There is also a very slight risk of infection at the puncture site.

What Do the Test Results Mean?

The normal amount of time it takes for blood plasma to clot is between 11 and 13.5 seconds if you are not taking blood-thinning medication. If you are taking this type of medication, it could take longer for a clot to form. If your blood clots within the normal amount of time, you probably do not have a bleeding disorder or you are taking the correct dose of blood-thinning medication.

If your blood does not clot in the normal amount of time, you may be on the wrong dose of warfarin or have liver disease, vitamin K deficiency, or a bleeding disorder such as Factor II deficiency. If you have a bleeding disorder, your doctor may recommend factor replacement therapy or a transfusion of blood platelets or fresh frozen plasma.

Was this article helpful? Yes No

Thank you.

Your message has been sent.

We're sorry, an error occurred.

We are unable to collect your feedback at this time. However, your feedback is important to us. Please try again later.

Show Sources

Trending Now

Understanding the Progression of Ankylosing Spondylitis
Understanding the Progression of Ankylosing Spondylitis
One serious potential cause of back pain is ankylosing spondylitis. Get an understanding of what this condition is, how it progresses, and potential complications in this slideshow.
How to Evaluate Your Multiple Sclerosis Treatment Plan
How to Evaluate Your Multiple Sclerosis Treatment Plan
Every multiple sclerosis (MS) patient is different, and no single treatment plan works for everyone. Learn more about what to consider when evaluating your MS treatment plan.
Easy Ways to Conceal an Epinephrine Shot
Easy Ways to Conceal an Epinephrine Shot
Learn how to discreetly carry your epinephrine autoinjectors safely and discreetly. It’s easier than you think to keep your shots on hand when you’re on the go.
The Best Multiple Sclerosis iPhone and Android Apps of the Year
The Best Multiple Sclerosis iPhone and Android Apps of the Year
These best multiple sclerosis apps provide helpful information and tools to keep track of your symptoms, including medication reminders.
Numbness, Muscle Pain and Other RA Symptoms
Numbness, Muscle Pain and Other RA Symptoms
The symptoms of RA are more than just joint pain and stiffness. Common symptoms include loss of feeling, muscle pain, and more. Learn more in this slideshow.