Synonyms: Prothrombin Time Assay; Protime; PT Why It Is Done • Determine a possible cause for abnormal bleeding or bruising. • Monitor the effects of the medication warfarin (Coumadin), which is used to prevent blood clots • Screen for deficiencies of certain blood clotting factors. • Screen for a vitamin K deficiency. Adequate • Monitor liver function. Test Overview Prothrombin time (PT) is a blood test that measures how long it takes blood to clot. A prothrombin time test can be used to screen for bleeding abnormalities. PT is also used to monitor treatment with medication that prevents the formation of blood clots. At least a dozen blood proteins, or blood clotting factors, are needed to clot blood and stop bleeding (coagulation). Prothrombin, or factor II, is one of several clotting factors produced by the liver. Adequate amounts of vitamin K are needed to produce prothrombin. Prothrombin time is an important coagulation test because it measures the presence and activity of five different blood clotting factors (factors I, II, V, VII, and X). How To Prepare No special preparation is necessary.