The blood's ability to form a fibrin clot through the extrinsic hemostatic pathway requires thromboplastin, calcium, and factors I, II, V, VII, and X. The Linear PT-HS reagent is a source of thromboplastin and calcium that specifically activates factor VII and the extrinsic pathway of coagulation. Factors of the intrinsic coagulation pathway (VIII, IX, and XII) are not involved, so deficiencies in these factors are not detected by the PT test.
