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INR Test

INR Test

The INR is a test of blood clotting, which is primarily used to monitor warfarin therapy, where the aim is to maintain an elevated INR in a certain range eg, 2.0 to 3.0. It is initially checked frequently, but as treatment is stabilised it may be done less often, eg fortnightly. Changes in the warfarin dose take several days to affect the INR result.

How the Test is Performed

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.

Medical Conditions and Symptoms

The INR is usually monitored as part of warfarin therapy, but it may also be checked by your doctor in relation to Liver Function Tests, because liver dysfunction can lead to decreased production of certain clotting factors.
The commoner reasons for warfarin therapy are:

  • Deep Venous 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 (LVF =Left Ventricular Failure or CCF =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.

Test Results Explained

The INR test result is given as a number. There are no units of measurement because the number is a ratio: the ratio of the sample’s Prothrombin Time (PT – a measure of clotting), to the Prothrombin Time of a normal sample of blood. A result of 1.0, up to 1.5, is therefore normal.
People on warfarin treatment will have different target INR ranges to aim for with warfarin treatment, depending on the reason for anticoagulation (blood-thinning treatment). One example is a range of 2.0 to 3.0 for DVT.
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”.
Warfarin doses are adjusted, initially every few days, aiming for the desired target range of INR. As treatment is stabilised it may be done less often, eg fortnightly. Changes in the warfarin dose take several days to affect the INR result. Patients on warfarin treatment will usually be advised by telephone by their doctor, or by the laboratory doing the INR test, on whether to change their warfarin dose, or exactly what dose to take, based on the INR result. 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