INR Test

Last updated: 24 November 2017

What does INR stand for?

INR stands for International Normalised Ratio, also referred to as Prothrombin time (PT), and is a standardised measurement of the time it takes for blood to clot.

What is an INR test?

An INR test measures how long it takes for your blood to clot. It is primarily used to diagnose unusual bleeding, blood clots, and monitoring people being treated with warfarin (an anti-clotting treatment).

Why get an INR test?

The most common reasons for an INR test are:

  • Monitoring as a part of warfarin therapy
  • In relation to liver function tests – liver dysfunction can lead to decreased production of certain clotting factors
  • Deep Vein Thrombosis (DVT) – a clot in a deep vein, commonly of the leg
  • Pulmonary Embolism (PE) – a clot in the lung, that has travelled through the veins (“embolised”) from a DVT formed elsewhere, such as the deep veins of the leg or pelvis
  • Atrial Fibrillation (AF) – an irregular heartbeat, sometimes accompanied by an enlarged left atrium – both of which predispose to the formation of blood clots in the heart, which may embolise to the blood vessels of the brain causing a cerebral infarction, a stroke – also known as a CerebroVascular Accident (CVA)
  • Some cases of Heart failure (Left Ventricular, and Congestive Cardiac Failure), especially when the heart is enlarged as in some forms of cardiomyopathy
  • Artificial heart valves of the mechanical type – because of the risk of a clot forming on the valve and causing a blockage in the heart

How an INR test is done?

The INR test is a blood test and requires a small tube of blood from a vein – approximately 4 millilitres. It is important that the tube is filled to the correct level, otherwise false results may occur.

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INR test results explained

The INR test result is given as a number, which is a ratio of:

  • The test sample’s Prothrombin time (a protein made by the liver and the time it takes to clot the blood)


  • The Prothrombin time of a normal sample of blood

A result of 1.0, up to 1.5, is therefore normal.

A low INR result means your blood is ‘not thin enough’ or coagulates too easily and puts you at risk of developing a blood clot.

A high INR result means your blood coagulates too slowly and you risk bleeding.

An INR lower than the desired range means the blood is “not thin enough” or clots too easily. An INR result higher than the desired range means the blood is “too thin”.

The result needs to be taken in context of recent INR measurements and dose changes. There are many medications that can affect the INR, and even a change in diet can result in changes to the INR – either raising or lowering it.

Related specialists

Related procedures

  • Blood Test (venesection)

Related tests

Also known as

  • Prothrombin Time (PT)
  • PT ratio



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This article is for informational purposes only and should not be taken as medical advice. If in doubt, HealthEngine recommends consulting with a registered health practitioner.

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