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

3-minute read

What is being tested?

An INR test measures the time for the 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.

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

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

Blood clotting helps the 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 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

Pathology Tests Explained 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 2020

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

Prothrombin time - Pathology Tests Explained

Describes how the prothrombin time (PT) test is used, why the PT test is done, and what the results might mean.

Read more on Pathology Tests Explained website

Coagulation Profile | HealthEngine Blog

Everything You Need to Know About Coagulation Profiles - What They Are, Why You Might Need One, How They're Performed and Test Results, Explained.

Read more on HealthEngine website

Coagulation factors - Pathology Tests Explained

Description of factors involved in coagulation of the blood, factors measured in the investigation of clotting disorders

Read more on Pathology Tests Explained 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

Anticoagulants | HealthEngine Blog

Anticoagulants are substances that work against the process of blood clotting. Normally, clotting only occurs when there is blood loss from a damaged blood vessel. However, there are several conditions that can cause blood clots to form in the absence of active bleeding. Currently there are two anticoagulants in use: warfarin and heparin.

Read more on HealthEngine website

D-Dimer Test & Results Explained | HealthEngine Blog

Everything You Need to Know About D-Dimer Tests - What They Are, Why You Might Need One, How They're Performed and Test Results, Explained.

Read more on HealthEngine website

Fibrinogen - Pathology Tests Explained

Explains how fibrinogen is used; when fibrinogen is used; what the results of fibrinogen might mean

Read more on Pathology Tests Explained 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 Victorian government logo Government of Western Australia, health department logo

Healthdirect Australia acknowledges the Traditional Owners of Country throughout Australia and their continuing connection to land, sea and community. We pay our respects to the Traditional Owners and to Elders both past and present.