The INR stands for International Normalised Ratio. It is a test used to check how long it takes for your blood to clot and it has been standardised so that tests done in different laboratories around the world on the same sample of blood will give the same result.

The INR for someone who is not taking warfarin is around 1. The higher your INR is, the longer it takes your blood to clot. The desired INR for you will depend on the reason for which you are taking warfarin.

Your doctor will try and keep your INR within a particular range called the target INR range.

The target INR range is different for different conditions. For example, for people with atrial fibrillation (an irregular heart beat in the top chambers of the heart) the target INR range is 2 to 3. Higher INR ranges are recommended for people who have a mechanical heart valve or for those with some irregular clotting conditions.

It is important to have regular blood tests to check that your INR is within the correct range. As the INR increases above the target range, the risk of bleeding increases. On the other hand, as the INR drops below the target range, the risk of developing a clot is increased.

Your doctor will tell you the correct INR range for you.

When you first start taking warfarin you will have your INR tested frequently, often every one or two days for the first week. Once your INR level has stabilised, testing will usually only be repeated every 4 weeks, or as often as your doctor tells you.

Every person requires a tailor-made dose, according to their medications, health and lifestyle. Two people taking warfarin for the same purpose may have very different doses, depending on how their body responds to warfarin. Your doctor will use your INR results to adjust your warfarin to the best dose for you.