Activated Partial Thromboplastin Time (aPTT) is a blood coagulation test that measures how long it takes for blood to clot through the intrinsic pathway. This test evaluates the function of coagulation factors VIII, IX, XI, XII, prekallikrein, and high molecular weight kininogen, which are essential for the contact activation pathway of blood clotting. The aPTT is particularly sensitive to deficiencies in these intrinsic pathway factors and is commonly used to screen for bleeding disorders like hemophilia A (factor VIII deficiency) and hemophilia B (factor IX deficiency). This test is also used to monitor unfractionated heparin therapy, as heparin prolongs the aPTT by enhancing antithrombin activity. Prolonged aPTT can indicate congenital clotting factor deficiencies, acquired factor deficiencies due to liver disease or vitamin K deficiency, or the presence of lupus anticoagulants. The aPTT is often ordered alongside the prothrombin time (PT) to provide a comprehensive evaluation of the coagulation system.