The INR test, also known as Prothrombin Time (PT), measures the time it takes for your blood to clot. It is used to monitor warfarin or similar anti-clotting medications. The PT test can also be used to check if you have a blood clotting problem associated with a blood disorder, liver damage or vitamin K status.

Why get tested?

The test measures how long it takes for your blood to begin to form clots. Prothrombin is a protein that acts as a clotting factor in your blood, helping it to stay at the right consistency. It is one of several clotting factors produced by the liver.

Clotting is caused by a series of chemical reactions called the coagulation cascade in which clotting factors are activated one after another to form a clot.  There must be a sufficient quantity of each clotting factor, and each must function properly, in order for normal clotting to occur. One of these steps is the conversion of prothrombin to thrombin.


What is the INR?

While the PT test measures the number of seconds it takes for a clot to form in your sample of blood, the INR is a calculation based on the results of the PT test.

The chemicals and instruments used to perform the PT test vary from one laboratory to another and give different results. To standardise results, in 1984 the World Health Organisation (WHO) developed the Internationalised Normalised Ratio (INR), a calculation based on the results of the PT test, to help work out the correct dosage for people on anti-coagulant medication.


Anticoagulant monitoring

If you are taking warfarin or a similar anti-clotting medication your doctor will check your INR regularly to make sure that your prescription is working properly. There is no set frequency for doing the test. Your doctor will order it often enough to make sure that your dosage is correct.

Having the right dosage of medication is important. It needs to achieve just the right balance – too much and there is a risk of internal bleeding, too little and there is more chance of your blood clotting.  The test is not helpful with monitoring the newer anticoagulants such as dabigatran, rivaroxaban or apixaban.

Having the test


Test preparation?
While there is no preparation needed before taking the test, some substances you consume — such as alcohol and different drugs, can interfere and give a misleading result. 

Antibiotics, aspirin and cimetidine can increase PT. 

Barbiturates, oral contraceptives and hormone-replacement therapy (HRT), and vitamin K - either in a multivitamin or liquid nutrition supplement - can decrease PT. 

The use of diuretics and antihistamines and the onset of illness or allergies, along withertain foods, such as beef and pork liver, green tea, broccoli, chickpeas, kale, turnip greens and soybean products contain large amounts of vitamin K and can alter PT results. 

Make sure that your doctor knows all the drugs you are taking so that the PT results are interpreted correctly.

Your results

Most people on warfarin medication have a target INR range of 2.0 to 3.0. This is a prothrombin time two to three times as long as that of a healthy person, using standardised conditions. For some people who have a higher risk of clotting, the INR needs to be higher – up to 3.0 to 4.0. Your doctor will use the INR to adjust the dose of your medication to get the PT into the range that is right for you.

The test result for PT depends on the method used. Results will be measured in seconds.

Anti-coagulant medication monitoring
People on anti-coagulant drugs usually have a target INR of 2.0 to 3.0 –a prothrombin time that is two to three times as long as in someone with a normal reading, For someone with a high risk of clot formation, the INR needs to be higher – up to 3.0 to 4.0.

Your doctor will use the INR to adjust the dose of your drug to get the PT into the range that is right for you. An increased prothrombin time or INR means that your blood is taking longer to form a clot. If you are not taking anti-coagulant drugs and your PT is prolonged, additional testing may be necessary to determine the cause.

Bleeding syndromes
The result of the PT is often interpreted with that of the Activated Partial Thromboplastin Clotting Time (APTT). APTT is another test that measures how long your blood takes to clot and can be used to look at how well the clotting factors are working. It is used if you have a bleeding disorder, such as von Willebrand disease or another disease that prevents your blood from clotting. This test is also used to monitor people on heparin therapy. Heparin is a blood thinner used to prevent dangerous blood clots.

Questions to ask your doctor

The choice of tests your doctor makes will be based on your medical history and symptoms.   It is important that you tell them everything you think may help.

You play a central role in making sure your test results are accurate. Do everything you can to make sure the information you provide is correct and follow instructions closely.

Talk to your doctor about any medications you are taking. Find out if you need to fast or stop any particular foods or supplements. These may affect your results. Ask:

  • Why does this test need to be done?
  • Do I need to prepare (such as fast or avoid medications) for the sample collection?
  • Will an abnormal result mean I need further tests?
  • How could it change the course of my care?
  • What will happen next, after the test?

More information

Pathology and diagnostic imaging reports can be added to your My Health Record.

You and your healthcare provider can now access your results whenever and wherever needed.Get further trustworthy health information and advice from healthdirect.

Last Updated: Thursday, 1st June 2023

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