The international normalized ratio or INR, is a calculated test that evaluates your coagulation. This test is based on prothrombin time that measures how long it takes your blood to form a clot. The INR test helps diagnose clotting or bleeding disorders and helps to monitor the effect of blood thinners or assess the risk of bleeding before surgery. 

Coagulation - where your blood turns from a liquid to a gel - is a process in which a blood clot is formed. In this process, there are several coagulation factors involved. When you, for example, cut yourself, the coagulation factors work together and prevent you from bleeding by forming a clot. How fast your blood clots depends on the amount of coagulation factors you have and whether they work correctly or not. In the case where your blood clots very quickly, you are in danger of forming blood clots which can cause a blockage inside your vessels and can cause, e.g., a heart attack, a stroke, or a pulmonary embolism. On the other hand, if your blood clots very slowly, you are more prone to bleeding too much after an injury. 

Whenever there are abnormalities in bleeding or clotting, the INR test helps doctors to determine more information about the underlying disorders. The higher the INR, the longer it takes your blood to form a clot, meaning you are more at risk of bleeding. Conversely, the lower your INR, the faster your blood can form dangerous blood clots. 

People whose blood is prone to excessive clotting often receive a blood-thinning medication called warfarin. Doctors use the INR to monitor the warfarin treatment and to prescribe the right dose. Since the INR is a calculated value, it is a parameter that allows doctors to directly compare results no matter the laboratory used to perform the test.