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An inactive plasma protein precursor of thrombin. Also known as factor II; thrombinogen.



(also factor II), in man and animals, a protein present in the blood plasma that is the primary component of the blood coagulation system.

Chemically, prothrombin is a glycoprotein composed of approximately 12 percent carbohydrates; the protein part of the molecule is in the form of one polypeptide chain. The molecular weight of prothrombin is approximately 70,000. Prothrombin is the precursor of the enzyme thrombin, which stimulates the formation of thrombi.

In an organism, prothrombin is activated into thrombin by prothrombinase, which is made up of phospholipid, Ca2+ ions, and coagulation factors V (accelerator globulin) and X (thrombokinase, autoprothrombin C). Factor X is a proteolytic enzyme and the active source of prothrombinase. Upon its conversion to thrombin, the prothrombin molecule loses approximately one-half of its former weight and three-quarters of its carbohydrate content.

The biosynthesis of prothrombin occurs in the cells of the liver and is regulated by vitamin K, which is produced by intestinal flora. In instances of vitamin K deficiency, the level of prothrombin in the blood drops below the normal level of 10 mg percent, often resulting in a tendency to bleed, which can manifest itself in early childhood hemorrhaging, obstructive jaundice, and certain diseases of the liver. It is believed that vitamin K deficiency causes the biosynthesis of anomalous molecules of prothrombin and reduces the ability of prothrombin to convert into thrombin. In medical practice, the prothrombin level is measured to determine the characteristics of an individual’s blood coagulation system (prothrombin index).


Magnusson, S. “Thrombin and Prothrombin.” In The Enzymes, 3rd ed., vol. 3. New York-London, 1971.


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The AvoSure PT-Pro+ provides efficient monitoring of patients on oral anticoagulant therapy by medical clinics or home healthcare nurses where high quality prothrombin time test results are needed in a timely manner.
The AvoSure PT-Pro provides efficient monitoring of patients on oral anticoagulant therapy to the doctor's office or home healthcare nurse where high quality prothrombin time test results are needed in a timely manner.
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Outsourcing prothrombin time tests was risking both legal liability and patient health.
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It has the potential to cut the number of prothrombin time tests that we do by more than 90%.

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