The blood's ability to form a fibrin clot through the intrinsic hemostatic pathway requires coagulation factors I, II, V, VIII, IX, X, XI, and XII, as well as platelet lipids and calcium. The APTT test is performed by adding to the plasma a suspension of rabbit brain cephalin containing a surface activator. APTT has been shown to be a simple and highly reliable measurement of the intrinsic coagulation mechanism.
