What is an international normalised ratio (INR) test?

The international normalised ratio (INR) test and prothrombin time (PT) are blood tests that check how long it takes for your blood to clot.

Prothrombin time (PT) measures in seconds how quickly your blood clots.

INR is a standard way to report PT results so they can be compared across different laboratories.

When is an INR test used?

INR tests are commonly used to monitor people taking some blood thinners (anticoagulant medicines), such as warfarin, to make sure they are taking the right dose. These medicines help prevent and treat blood clots.

Your doctor may prescribe warfarin to:

• prevent or treat blood clots in conditions, such as deep vein thrombosis and pulmonary embolism
• reduce the chance of blood clots in people with mechanical heart valves or certain heart conditions
• lower your risk of stroke if you have atrial fibrillation

Your INR results help your doctor to adjust your warfarin dose. Too much warfarin can increase your bleeding risk, while too little may not prevent blood clots. This is called your 'target INR range'.

INR testing can also help diagnose and monitor:

• abnormal blood clotting or bleeding conditions
• vitamin K deficiency
• liver disease
• how long it takes for your blood to clot, in preparation for surgery

Is INR testing needed when taking all types of blood thinners?

Not all blood thinners need INR monitoring.

Some newer anticoagulants, called non-vitamin K oral anticoagulant medicines (NOACs) or direct oral anticoagulant medicines (DOACs), do not need regular INR testing. While newer blood thinners are available, warfarin is still preferred for some people.

Talk to your doctor or pharmacist about which blood thinner is right for you and whether you need an INR blood test.

Where can I get an INR test?

You usually need to go to a pathology collection centre with a referral from your doctor. Your doctor will recommend one near you.

Sometimes, your doctor will collect blood for testing in their clinic. Blood tests are also routinely done in hospitals.

How do I prepare for an INR test?

There is usually no special preparation needed for an INR test.

Some foods, such as cranberry, and medicines, such as antibiotics and anti-inflammatory medicines, can affect your test results. Make sure to tell your doctor about all your medicines to make sure your results are correct. You should also tell your doctor about any significant dietary changes, if you are having INR monitoring while on warfarin.

If you are having other blood tests done at the same time, you may need to prepare. Check with your doctor or the pathology collection centre if there is anything you need to do before your blood test.

What happens during an INR test?

A health professional will take a sample of blood, usually from a vein in your arm. The blood will then be sent to a laboratory for testing.

What do my INR test results mean?

If your INR test result is above your target range, it means your blood takes too long to clot. You may have a higher risk of bleeding.

If your INR test result is below your target range, it means your blood clots too quickly. You may have a higher risk of developing a blood clot.

Many things can affect your INR result. Make sure to tell your doctor if you:

• take any medicines
• take any supplements
• have recently changed your diet

• If you had your INR tested, but don't take an anticoagulant medicine, ask your doctor what your results mean in your personal situation.

What happens if my INR is too high or too low?

Warfarin helps prevent blood clots, but too much or too little can be dangerous.

If your INR is too high, you are at risk of serious bleeding. Your doctor may:

• lower your warfarin dose
• ask you to stop taking warfarin for a short time
• give you medicine to reverse the effects of warfarin
• monitor you closely until your risk of bleeding is reduced

If your INR is too low, your blood may clot too easily, and your risk of blood clots is increased. Your doctor will advise you what dose you should take to bring your INR back to your target range.

Resources and support

Pathology Tests Explained has more information about Prothrombin time and INR testing.

Read our Guide to blood testing to learn more about blood tests, including:

• what to consider before having the test
• what happens during a blood test
• how accurate results are
• blood test costs

You can also call the healthdirect helpline on 1800 022 222 (known as NURSE-ON-CALL in Victoria). A registered nurse is available 24 hours a day, 7 days a week.

Other languages

• NSW Government offers information about warfarin in a variety of community languages.