Prothrombin time (PT) test & INR

A blood clotting test

A prothrombin time (PT) test is a blood test that measures how long it takes your blood to clot.

Key points

  1. It is used to check for blood disorders and to monitor medicines taken to prevent blood clots.
  2. Results are commonly measured in seconds or as a ratio known as the international normalised ratio (INR).
  3. PT and INR are both measures of how long it takes your blood to clot.
  4. If you are taking a blood-thinning medicine like warfarin, you will need regular PT/INR tests.
  5. Your doctor may adjust your medicine based on your PT/INR results.

Who needs a PT/INR test?

If you take a blood-thinning medicine like warfarin to prevent clots forming, your doctor will recommend a PT/INR test to check the medication is working effectively.

A PT/INR test can also be requested in people not taking anticoagulants. This is to check for bleeding disorders and liver problems.

How do I prepare for a PT/INR test?

A blood sample is needed for a PT/INR test. For the most part, you don't need to do anything before having the test. 

However, some medicines may change the result of these tests. Tell your doctor about all prescription and non-prescription medication (such as herbal products) you take.

How is the blood sample collected for PT/INR testing?

A blood sample is is taken by a needle placed in a vein in your arm.

  • Before the needle is inserted, an elastic band is wrapped around your upper arm.
  • It may feel tight for a few seconds.
  • You may feel nothing at all from the needle, or you may feel a small brief sting or pinch.
  • The blood sample is collected in a tube, which is sent to the laboratory for analysis.

In some cases, portable INR monitors (called portable coagulometers) are used to measure INR (see below).

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

How often is a PT/INR test needed?

If you are taking warfarin, you will need regular PT/INR tests to monitor your response to the medicine.

  • At first prothrombin time (measured as INR) will be checked every day until it is in the correct range.
  • Once stable, your INR will be measured less often, such as every 1–2 weeks.
  • After that, if it is stable, it may be stretched out to every 4–8 weeks.

What do the results of my PT/INR test mean?

Prothrombin time test results can be presented either in seconds or as a ratio (INR).

Prothrombin time in seconds

Prothrombin time is usually measured in seconds, that is, the time it takes for your blood to clot. 

The results of the prothrombin test vary depending on the laboratory, the test chemicals used in different laboratories and the method used to test blood.

  • A number higher than average means it takes blood longer than usual to clot.
  • A lower number than average means your blood clots more quickly than expected.
  • A prolonged PT means that the blood is taking too long to form a clot.
  • This may be caused by conditions such as liver disease, vitamin K deficiency or a coagulation factor deficiency.

Prothrombin time as a ratio – INR

Because the results of the prothrombin time test vary from laboratory to laboratory, a ratio called the international normalised ratio (INR) is calculated. The INR is a formula that allows for differences in different laboratories so that test results can be compared.   

The INR target range is different for different people taking warfarin, depending on the condition for which the warfarin is being prescribed.

In most situations, the target INR is between 2–3. For some people who have a high risk of a blood clot, the INR needs to be higher – about 2.5–3.5.

  • If your INR is above the target range, this means that your blood is clotting too slowly and you are at risk of bleeding.
  • If your INR is below the target range, this means that your blood is clotting more quickly than ideal (for people on warfarin) and you may be at risk of unwanted clots being formed. 
  • When you INR is out of range, your doctor will advise you on how to adjust your warfarin dose.   

Learn more

The following links provide more information on the PT test and INR. Be aware that websites from other countries may contain information that differs from New Zealand recommendations.

Lab Tests Online Australasian Association of Clinical Biochemists
What is a prothrombin time test? WebMD, US

Credits: Health Navigator Editorial Team. Reviewed By: Gwenda Lawrence, medical laboratory scientist, Auckland