Prothrombin time (PT) evaluates the ability of blood to clot properly, it can be used to help diagnose bleeding. The PT may be ordered when a patient who is not taking anti-coagulant drugs has signs or symptoms of a bleeding disorder, which can range from nosebleeds, bleeding gums, bruising, heavy menstrual periods, blood in the stool and/or urine to arthritic-type symptoms (damage from bleeding into joints), loss of vision, and chronic anemia. Sometimes the PT may be ordered when a patient is to undergo an invasive medical procedure, such as surgery, to ensure normal clotting ability.
The International Normalized Ratio (INR) is used to monitor the effectiveness of blood thinning drugs such as warfarin (Coumadin). These anti-coagulant drugs help inhibit the formation of blood clots. They are prescribed on a long-term basis to patients who have experienced recurrent inappropriate blood clotting. This includes those who have had heart attacks, strokes, and deep vein thrombosis (DVT). Anti-coagulant therapy may also be given as a preventative measure in patients who have artificial heart valves and on a short-term basis to patients who have had surgeries, such as knee replacements. The anti-coagulant drugs must be carefully monitored to maintain a balance between preventing clots and causing excessive bleeding.
Information provided by the American Association of Clinical Chemistry site here (1-2010).