Prothrombin Test (PT), also known as Pro Time Test, is a blood test that measures how long it takes for blood to clot. Often used for identifying bleeding problems, it is also used to check if medicines (mostly Warfarin) that prevent blood clots are effectively working. A partial thromboplastin time (PTT) test measures the time it taken for a blood clot to form. Normally, when you get a cut or injury that causes bleeding, proteins in your blood called coagulation factors work together to form a blood clot. The clot stops you from losing too much blood.