Healthdirect Free Australian health advice you can count on.

Medical problem? Call 1800 022 222. If you need urgent medical help, call triple zero immediately

healthdirect Australia is a free service where you can talk to a nurse or doctor who can help you know what to do.

beginning of content

International normalised ratio (INR) test

2-minute read

An INR test measures the time for your blood to clot. It is also known as prothrombin time, or PT. It is used to monitor blood-thinning medicines, which are also known as anticoagulants. The INR, or international normalised ratio, can also be used to check if you have a blood clotting problem.

What is being tested?

Prothrombin is a protein produced by the liver. It is one of several proteins, known as clotting factors, that help your blood stay at the right consistency. The INR test measures this clotting factor by measuring how long it takes for your blood to clot.

Why would I need this test?

Blood-thinning medicines such as warfarin are used to prevent blood clots. Warfarin is usually prescribed for people who have atrial fibrillation (where the heart beats unevenly) or who have had artificial valves fitted. How well warfarin is working is measured by how it increases the blood clotting time. So if you are taking warfarin you might need an INR test.

Blood thinning medicines don’t actually make your blood thinner. They work by increasing the time it takes for your blood to clot.

Your doctor can use an INR test to make sure the warfarin dose is right for you.

Blood clotting helps your body stop bleeding. But blood clots can also cause strokes or heart attacks. The INR test helps balance the risk of internal bleeding against the risk of blood clotting.

This test can also be used to:

  • check for bleeding conditions
  • check for blood clotting problems, especially before surgery
  • check how well your liver is working

How to prepare for this test

No special preparation is needed. However many medicines, foods and illnesses can affect this test. So tell your doctor about any medicines and supplements you are taking, as well as any changes in your diet.

Understanding your results

When you are having warfarin treatment, your doctor will probably have a target INR for you. If your INR is too high, it might mean your blood clots too slowly, and there is a risk of you bleeding. If your INR is too low, it might mean you have a risk of developing a blood clot.

If you are not having warfarin treatment and your INR is higher than normal, you might need further tests.

More information

Lab Tests Online has more information about INR testing. You can also read more about blood tests in general.

About blood testing

Visit our ‘Guide to blood testing’ to learn more about blood tests in general with information such as:

  • what to consider before having the test
  • what happens during a blood test
  • results accuracy
  • blood tests cost

Learn more here about the development and quality assurance of healthdirect content.

Last reviewed: August 2018

Back To Top

Need more information?

These trusted information partners have more on this topic.

Top results

CoaguChek device (INR test machine) information | myVMC

The CoaguChek(R) monitor is a hand-held INR test machine that is used to measure blood clotting by home INR testing in patients taking warfarin.

Read more on myVMC – Virtual Medical Centre website

Blood Clotting: International Normalised Ratio (INR) | myVMC

The international normalised ratio (INR) is a laboratory measurement used to determine the effects of oral anticoagulants such as warfarin on the body's blood clotting.

Read more on myVMC – Virtual Medical Centre website

What the 6 most common pathology tests reveal about your health | | Know Pathology Know Healthcare

What the 6 most common pathology tests reveal about your health Apr 1, 2016 0 Comment Post By:Annette Stenhouse Over 500 million pathology tests were ordered for Australians in the last 12 months, covering a vast array of diseases and conditions from cholesterol checks to cancer diagnoses

Read more on Know Pathology Know Healthcare website

Healthdirect 24hr 7 days a week hotline

24 hour health advice you can count on

1800 022 222

Government Accredited with over 140 information partners

We are a government-funded service, providing quality, approved health information and advice

Australian Government, health department logo ACT Government logo New South Wales government, health department logo Northen Territory Government logo Government of South Australia, health department logo Tasmanian government logo Government of Western Australia, health department logo