coagulation factors factors essential to normal blood
clotting, whose absence, diminution, or excess may lead to abnormality of the clotting. Twelve factors, commonly designated by Roman numerals, have been described (I–V and VII–XIII; VI is no longer considered to have a clotting function). (See table 6.)
Factor I is a high-molecular-weight plasma protein that is converted to fibrin through the action of thrombin; deficiency conditions are called
afibrinogenemia and
hypofibrinogenemia. Called also
fibrinogen.
Factor II is a glycoprotein present in the plasma that is converted into
thrombin in the
common pathway of coagulation; deficiency is called
hypoprothrombinemia. Called also
prothrombin.
Factor III is involved in the
extrinsic pathway of coagulation, activating factor X; called also
tissue thromboplastin or
factor.
Factor IV is
calcium, required in many stages of blood clotting.
Factor V is a heat- and storage-labile material, present in plasma and not in serum and is involved in the
intrinsic and
extrinsic pathways of coagulation, causing the cleavage of
prothrombin to the active
thrombin. Deficiency causes
parahemophilia. Called also
accelerator globulin or
factor and
proaccelerin.
Factor VI is no longer considered in the scheme of hemostasis, and hence is assigned neither a name nor a function.
Factor VII is a heat- and storage-stable material, present in serum and in plasma and participating in the
extrinsic pathway of coagulation, acting with factor III to activate factor X. Deficiency, either hereditary or acquired (
vitamin k deficiency), leads to hemorrhagic tendency. Called also
proconvertin and
serum prothrombin conversion accelerator (SPCA).
Factor VIII is a relatively storage-labile material that participates in the
intrinsic pathway of coagulation, acting as a cofactor in the activation of factor X. Deficiency, an X-linked recessive trait, results in
hemophilia a (classical hemophilia). Called also
antihemophilic factor (AHF) and
antihemophilic globulin (AHG).
Factor IX is a relatively storage-stable substance involved in the
intrinsic pathway of coagulation, acting to activate factor X. Deficiency of this factor results in a hemorrhagic syndrome called
hemophilia b (or Christmas disease), which is similar to classical hemophilia A. It is treated with purified preparations of the factor, derived from human plasma or recombinant, or with
factor IX complex. Called also
plasma thromboplastin component (PTC) and
antihemophilic factor B.
Factor X is a heat-labile material with some storage stability, which is involved in both
intrinsic and
extrinsic pathways of coagulation, uniting them to begin the
common pathway. Once activated, it complexes with calcium, phospholipid, and activated factor V to form
prothrombinase, which cleaves and activates
prothrombin to
thrombin. Called also
Stuart or
Stuart-Prower factor.
Factor XI is a stable factor involved in the
intrinsic pathway of coagulation, activating factor IX. Deficiency results in
hemophilia c. Called also
plasma thromboplastin antecedent (PTA) and
antihemophilic factor C.
Factor XII is a stable factor activated by contact with glass or other foreign substances, which initiates coagulation through the
intrinsic pathway by activating factor XI; called also
Hageman factor.
Factor XIII is a factor that polymerizes
fibrin monomers, enabling fibrin to form a firm blood clot. Deficiency causes a clinical hemorrhagic diathesis. Called also
fibrin-stabilizing factor.