The Full form of PR is Prothrombin Ratio. It is also called the International Normalized Ratio. PR is a standardized number that’s figured out in the lab. Blood clotting or Coagulation is a protective mechanism of the body against bleeding. When damage occurs to a blood vessel in human body, a series of reactions take place involving substances found in the blood known as clotting factors. They are called clotting factors because they contribute to the formation of a blood clot. If a person takes blood thinners, also called anticoagulants or anti-clotting medicines, it’s especially important to check his PR. The INR is figured out using the results of the Prothrombin Time (PT) test. This measures the time it takes for your blood to clot. The INR is an international standard for the PT. A person may need this test if he takes medicines, such as warfarin, that change the way your blood clots. Anti-clotting medicines are helpful if a person is at risk for a heart attack or stroke. But if anyone is in an accident and his blood doesn’t clot quickly, he may be at risk of dangerous bleeding. His healthcare provider uses the PR to know whether his anti-clotting medicines are on target or whether he needs a change in his dosage.