prothrombin

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Protein in plasma involved in coagulation of blood. The prothrombin time is an index of the coagulability of blood (and hence of vitamin K nutritional status) based on the time taken for a citrated sample of blood to clot when calcium ions and thromboplastin are added.

A glycoprotein precursor of thrombin that is produced in the liver and is necessary for the coagulation of blood. A prothrombin deficiency is uncommon but may occur in liver disease. Vitamin K is essential for the synthesis of prothrombin.

A glycoprotein present in the plasma that is converted into thrombin by extrinsic thromboplastin during the second stage of blood clotting; called also clotting factor II.

• p. consumption test — determines thromboplastin generating capacity, which provides information about the first stage of coagulation. When clotting of a normal blood sample occurs, prothrombin is converted to thrombin, thus there should be little or no prothrombin in the serum after the clot is formed. If, however, there is deficiency of blood coagulation, some of the prothrombin will not be utilized (consumed). Abnormal results of the test are found in deficiencies of the first-stage factors of coagulation (factors VIII and IX), and in the presence of circulating anticoagulants, thrombocytopenia, and any other condition leading to inadequate generation of thromboplastin. Called also serum clot time.
• p. converting activity — the step in blood coagulation in which factor Xa, together with factor V react with phospholipid in the presence of calcium, activates prothrombin to form thrombin.
• p. deficiency — see hypoprothrombinemia.
• p. time test — a test to measure the activity of clotting factors V, VII and X, prothrombin and fibrinogen. Deficiency of any of these factors leads to a prolongation of the one-stage prothrombin times, as will circulating anticoagulants that are active against factors V, VII, or against thromboplastin. Called also pro-time.
— The one-stage (OSPT) test is performed by measuring the time required for clot formation after tissue extract and calcium are added to citrated plasma. Called also Quick's prothrombin test. A two-stage test determines plasma levels of prothrombin by finding the dilution of plasma that clots a standard fibrinogen reagent in a set period of time.