Prothrombin Time (PT), Serial is a specialized test that measures blood clotting time and calculates the International Normalized Ratio (INR) to monitor anticoagulant therapy. This test evaluates the extrinsic pathway of blood coagulation by measuring the time it takes for blood to clot when tissue factor is added. The PT test is particularly valuable for monitoring patients on warfarin or other vitamin K antagonist medications, as it helps ensure the medication is working effectively without causing excessive bleeding or clotting. Serial measurements allow healthcare providers to track changes over time and adjust medication dosages as needed. This test is essential for patients with conditions requiring long-term anticoagulation, such as atrial fibrillation, deep vein thrombosis, or mechanical heart valves.