Fibrinogen is a protein involved in clotting. When the body is injured, it is converted by thrombin to fibrin, which starts the clotting process. Inflammation and injury also encourage the liver to create more fibrinogen. This process starts a feedback loop as the increase in fibrin reduces the activity of thrombin and stops the conversion of fibrinogen. If this feedback is interrupted, usually by a genetic condition, clotting can continue to increase leading to blood clots and thrombosis.
Fibrinogen can be measured directly in blood tests and can be an indicator of many clotting disorders, although high levels may merely indicated bleeding or injury. Fibrinogen also increases in pregnancy.