The PT INR (Prothrombin Time International Normalized Ratio) is a medical test used to assess the clotting ability of a person's blood. It is primarily used to monitor and adjust the dosage of anticoagulant medications, such as warfarin (Coumadin), which are commonly prescribed to prevent blood clots in conditions like atrial fibrillation, deep vein thrombosis (DVT), and pulmonary embolism.