Prothrombin time (PT)

What Is Prothrombin Time (PT Test)?

The prothrombin time (PT) test is a blood test, used to determine how long it takes for the blood to clot. It is also commonly referred to as the international normalized ratio (INR) test. The test measures the time needed for clot formation and the strength of the clot. The test helps doctors determine how well a person’s blood is clotting.

Why Is the Prothrombin Time Test Carried Out?

The test is used to assess a number of health problems, including:

  • liver disease
  • vitamin K deficiency
  • circulation problems
  • blood clotting disorders
  • bleeding disorders

The prothrombin time test is also used to monitor people who are taking the anticoagulant (blood thinner) medication warfarin. Warfarin is prescribed to people with atrial fibrillation and those who’ve had a heart attack, stroke, and certain types of heart valve replacement.

How Is the Test Prepared for?

Doctors will usually provide information about how to prepare for the prothrombin time test. The person having the test may need to:

  • stop taking warfarin for a few days before the test
  • stop taking certain other medications, depending on the person's medical condition
  • avoid eating certain foods, depending on the person's medical condition
  • fast for a certain amount of time before the test

In most cases, the person will be able to drink water while fasting. A doctor or nurse should give the person detailed instructions about what they should and shouldn’t do before the test.

Prothrombin Time Test Procedure

The prothrombin time test is a simple and quick procedure that involves taking a blood sample. It is usually performed by a doctor, nurse, or phlebotomist (a specialist in taking blood samples). The person having the test will need to sit or lie down while the blood sample is taken.

To take the blood sample, the person will have their skin disinfected with antiseptic liquid. An area on the arm is then located and a needle is inserted into a vein. The sample can then be taken.

Types of Prothrombin Time Tests

For some prothrombin time tests, a liquid called a “reagent” is added to the person’s blood sample. The reagent contains thromboplastin, a molecule found in the lining of blood vessels, which triggers the clotting process.

Another type of prothrombin time test is called the APTT (activated partial thromboplastin time) test. This test involves adding a special mixture of proteins and phospholipids to the blood sample.

Prothrombin Time Test Risks

As with any procedure that involves taking blood, there's a very small risk of feeling faint when the needle is inserted. The person having the test might feel a mild to moderate amount of discomfort when the needle is being inserted and when the blood is being taken.

In very rare cases, an infection can occur at the site. If this does occur, antibiotics may be prescribed.

Prothrombin Time Test Results

Once the sample has been taken, it’s sent to a laboratory for testing. Laboratory technicians will measure the time it takes for the person’s blood to clot, compared to a sample of normal blood. The results of the test will usually be available within 1-2 days.

The normal range for prothrombin time is 11–13.5 seconds. Results that are higher than this suggest that the person’s blood is excessively thin and the risk of bleeding is high. Lower-than-normal results suggest a faster-than-normal clotting process, which could result in blood clots forming and blocking the flow of blood.

When Is a Prothrombin Time Test Taken?

The prothrombin time test is often taken when there are signs or symptoms of a blood clotting disorder. It is also used to monitor people taking warfarin, as well as diagnose and monitor the treatment of liver disease. The test may also be used to determine how well a person is responding to certain medications.