Partial Thromboplastin Time (PTT) is a laboratory test that measures the effectiveness of the blood clotting system by determining the time it takes for a clot to form after adding a substance that activates plasma. This test is primarily used to diagnose and monitor bleeding and clotting disorders, such as hemophilia, and is considered an essential tool for monitoring patients receiving anticoagulant treatment, such as heparin. PTT is also part of a comprehensive assessment of coagulation functions and is often used alongside other tests, such as Prothrombin Time (PT), to provide accurate information about the health of the blood clotting system.