The Activated Partial Thromboplastin Time (APTT) test measures how long it takes for your blood to clot. This test evaluates the function of several blood clotting factors in the intrinsic and common pathways of coagulation. Prolonged APTT can indicate bleeding disorders, the presence of inhibitors, or issues with blood clotting factors. It is often used to monitor patients on anticoagulant therapy or diagnose clotting disorders.