The factor I of coagulation, or fibrinogen, is a protein in blood plasma. Fibrinogen is converted to fibrin by the action of another protein, thrombin. This fibrin is involved in the formation of a blood clot during the phenomenon of coagulation, which stops bleeding. Its blood levels may vary in certain conditions. In case of an increase, many diseases can be suspected such as infection, cancer, lymphoma, or inflammatory diseases. Its decrease may be due to liver failure since part of its synthesis comes from the liver.