What is an INR?

INR stands for International Normalised Ratio, but what is an INR?

To explain,

The number of seconds that it takes for a sample of blood to clot is known as the Prothrombin time (PT). When testing of blood samples is carried out in the laboratory the PT recorded can vary depending on the chemicals used and the conditions in the lab.  This can make PT results hard to interpret.

INR is a method of reporting the result of a PT test in a standard way so that blood test results can easily be compared no matter which laboratory or hospital has run the test.

The rate at which blood clots in a person who does not use anticoagulant drugs, is reported as an INR of 1. Someone who takes Warfarin will be likely to have an INR of around 2-3.  This means that their blood will clot 2 to 3 times slower than normal.

For each individual who is prescribed Warfarin the doctor, or anticoagulant specialist will decide on the correct INR level for that person. This will usually be a specific range that the INR should be kept within and is often referred to as the Therapeutic Range.  The doctor will try and establish a level that will slow down the clotting of the blood sufficiently to stop unwanted clots occurring but care must be taken not to slow it down too much or the patient will be at risk of bleeding.what is an inr