A prothrombin time (PT) test measures how long it takes for a clot to form in a blood sample. An INR (international normalized ratio) is a type of calculation based on PT test results. Prothrombin is a protein made by the liver. It is one of several substances known as clotting (coagulation) factors. When you get a cut or other injury that causes bleeding, your clotting factors work together to form a blood clot. Clotting factor levels that are too low can cause you to bleed too much after an injury. Levels that are too high can cause dangerous clots to form in your arteries or veins. A PT/INR test helps find out if your blood is clotting normally. It also checks to see if the medicine that prevents blood clots is working the way it should. Other names: prothrombin time/international normalized ratio, PT protime.