The APTT in contrast to the PT, measures the activity of the intrinsic and common pathways of coagulation. The division of the clotting cascade into the intrinsic, extrinsic and common pathways has little in vivo validity but remains a useful concept for interpreting the results of laboratory investigations. The term 'thromboplastin' in this test refers to the formation of a complex formed from various plasma clotting factors which converts prothrombin to thrombin and the subsequent formation of the fibrin clot. The term 'Activated Partial Thromboplastin Time (APTT)' derives from the original form of the test (devised in 1953) in which only the phospholipid concentration of the test was controlled (as opposed to the phospholipid and the surface activator concentrations) and the name 'partial thromboplastin' was applied at the time to phospholipid preparations which accelerated clotting but did not correct the prolonged clotting times of haemophilic plasma. Essentially the term 'partial' means phospholipid is present but no Tissue Factor.
Activated Partial Thromboplastin Time [APTT]