Prothrombin Time: About This Test

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What is it?

Prothrombin time (PT) is a blood test that measures how long it takes for blood to clot. Prothrombin is one of several clotting factors your body makes to help your blood form clots when a blood vessel is damaged.

A PT test may also be called an INR test (for international normalized ratio).

Why is this test done?

The test can be used to check for bleeding problems. It is also used to check how well warfarin, a medicine to prevent blood clots, is working.

How can you prepare for the test?

  • In general, you don't need to prepare before having this test. Your doctor may give you some specific instructions.

What happens during the test?

  • A health professional takes a sample of your blood.

What else should you know about the test?

  • Your results will include an explanation of what a "normal" result is. This is called a "reference range." It is just a guide. Your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed may still be normal for you.
  • If you're taking warfarin, regular testing helps your doctor make sure you're taking the right amount.

How long does the test take?

  • The test will take a few minutes.

What happens after the test?

  • You will probably be able to go home right away.
  • You can go back to your usual activities right away.

Follow-up care is a key part of your treatment and safety. Be sure to make and go to all appointments, and call your doctor or nurse call line if you are having problems. It's also a good idea to keep a list of the medicines you take. Ask your doctor when you can expect to have your test results.

Where can you learn more?

Go to https://www.healthwise.net/patientEd

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