A prothrombin time test measures how quickly your blood clots. Sometimes called a pro time test or PT test, a prothrombin time test uses a sample of your blood.Prothrombin is a protein produced by your liver that helps your blood to clot. When you bleed, a series of chemicals (clotting factors) activate in a stepwise fashion. The end result is a clot which stops the bleeding. One step in the process is prothrombin turning into another protein called thrombin. The prothrombin time test measures how well the clotting process works and how long it takes to occur.