Prothrombin time (PT) is determined through a laboratory blood test that measures the time it takes for a person's blood to clot. A sample of blood is mixed with a reagent that triggers the clotting process. The time it takes for the clot to form is measured, and the result is compared to a control sample. The PT result is typically reported in seconds. To standardize results across different laboratories and reagents, the International Normalized Ratio (INR) is calculated based on the PT value. INR allows for consistent monitoring of clotting tendencies, especially important for individuals on anticoagulant therapy, ensuring their blood clotting remains within a safe and therapeutic range.