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. It is used to check for blood disorders and to monitor medicines taken to prevent blood clots. Results are commonly measured in seconds or as a ratio (INR).

What are PT & INR?

PT and INR are both measures of how long it takes your blood to clot, expressed in two different ways. 

  • PT stands for prothrombin time. It is a measure in seconds of how long it takes your blood to clot.
  • The results of the PT test vary depending on the laboratory, the test chemicals used in different laboratories and the method used to test blood. 
  • To standardise the PT test, a ratio called the international normalised ratio (INR) is calculated.
  • The INR is a formula that allows for differences between laboratories so that test results can be compared.  

Who needs a PT/INR test?

If you take a blood-thinning medication such as warfarin to prevent clots from 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, to check for bleeding disorders, and liver problems.

How to prepare for a PT/INR test

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

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 testing?

A blood sample is is taken by a needle placed in a vein in the 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, the health professional will clean your hand, use a lancet to puncture the skin, and place a small tube on the puncture site in order to collect your blood.

How often is a PT/INR test required?

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

  • Initially, prothrombin time (measured as INR) will be checked daily until it is in the correct range.
  • Once stable, your INR will be measured less often such as every 1 to 2 weeks, and then it may be stretched out to every 4 to 8 weeks if stable. 

Results of PT/INR test

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 ie, 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.

What do your PT test results mean?
  • A number higher than average means it takes blood longer than usual to clot. A lower number means 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.    

What do your INR results mean?

For people taking warfarin, the INR target range will be different for different people, depending on the condition for which the warfarin is being prescribed.  In most situations, the target INR is between 2 to 3. For some who have a high risk of a blood clot, the INR needs to be higher – about 2.5 to 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.

Blood test safety information Labtests NZ
Lab tests Online  Australasian Association of Clinical Biochemists

Credits: Editorial team.