thromboplastin

n.
A protease that converts prothrombin to thrombin in the early stages of blood clotting. Also called thrombokinase.

(throm′bōplas′tin)
n

A substance necessary to the coagulant activity of tissue extracts; also has been referred to as the direct activator of prothrombin and as a substance from plasma, platelets, and tissues that initiates thromboplastic activity in blood coagulation. See also thromboplastin, extrinsic.

A substance in blood and tissues which, in the presence of ionized calcium, aids in the conversion of prothrombin to thrombin. Extrinsic and intrinsic thromboplastin are formed as the result of the interaction of different clotting factors; the factors that combine to form extrinsic thromboplastin are not all derived from intravascular sources, whereas those that form intrinsic thromboplastin are.

• activated partial t. time — see activated partial thromboplastin time.
• extrinsic t. — the prothrombin activator formed as a result of interaction of coagulation factors III, VII, and X which, with factor IV, aids in the formation of thrombin.
• t. generation time (TGT) — evaluates the first stage in blood coagulation by measuring the efficiency of prothrombinase formation.
• intrinsic t. — the prothrombin activator formed as a result of interaction of coagulation factors V, VII, IX, X, XI and XII and platelet factor 3 (PF-3), which, with factor IV, aids in the conversion of prothrombin to thrombin.
• plasma t. antecedent (PTA) — clotting factor XI; deficiency occurs in cattle and dogs, causing mild to severe bleeding tendencies called hemophilia C.
• plasma t. component (PTC) — clotting factor IX; deficiency causes christmas disease. Called also Christmas factor, antihemophilic factor B, autoprothrombin II.
• t. time — see activated partial thromboplastin time.
• tissue t. — factor III, a material derived from several sources in the body (e.g. brain, lung), and is important in the formation of extrinsic prothrombin converting principle in the extrinsic pathway of blood coagulation. Called also tissue factor.