Prothrombin Time (PT) test is a blood test used to evaluate the ability of the blood to clot. The test measures the time it takes for the blood to form a clot after a specific substance, called a reagent, is added to the sample. The test is often used to monitor the effectiveness of blood-thinning medications, such as warfarin, and to diagnose bleeding disorders such as hemophilia or von Willebrand disease. The normal range for PT is typically between 11 and 13 seconds, although this can vary depending on the laboratory and the reagents used. Results that fall outside the normal range may indicate a problem with blood clotting and may require further testing or medical treatment.