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Prothrombin Time: About This Test

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 do you prepare for the test?

In general, there's nothing you have to do before this test, unless your doctor tells you to.

How is the test done?

A health professional uses a needle to take a blood sample, usually from the arm.

In some cases, the health professional will take a sample of blood from your fingertip instead of your vein. For a finger stick blood test, the health professional will clean your hand, use a lancet to puncture the skin, and place a small tube on the puncture site to collect your blood.

Some people use a monitor at home to test a small blood sample.

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. It depends on the reason for the test.
  • 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 advice line (811 in most provinces and territories) 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

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