A prothrombin time (PT) blood test is a laboratory test that measures the time it takes for blood to clot. The test is used to assess the function of the blood clotting system and to monitor the effectiveness of blood-thinning medications, such as warfarin. The PT test measures the activity of prothrombin, a protein that is needed for blood to clot. The test is typically done by drawing a small amount of blood from a vein in the arm and sending it to a laboratory for analysis. Abnormal results on a PT test may indicate a problem with the blood clotting system, such as a deficiency of clotting factors or the presence of an inhibitor that prevents blood from clotting properly. The PT test is often ordered along with other tests, such as the international normalized ratio (INR) test, to help diagnose and monitor conditions affecting the blood clotting system.