The activated partial thromboplastin time (APTT) measures how long it takes for a clot to form in a plasma sample when the intrinsic coagulation pathway is activated. It assesses the function of clotting factors involved in the intrinsic and common pathways — specifically factors XII, XI, IX, VIII, X, V, II (prothrombin), and fibrinogen.

APTT is used to investigate unexplained bleeding (bruising, prolonged bleeding from cuts, haemarthrosis), diagnose clotting factor deficiencies including haemophilia A (factor VIII) and haemophilia B (factor IX), and monitor heparin anticoagulation therapy. A prolonged APTT indicates impaired clot formation; a shortened APTT may suggest a hypercoagulable state.