Fibrinogen

Fibrinogen is the clotting element present in blood. When blood comes out of a wound in a vessel, the fibrinogen is activated and forms fibrin, which clots the blood. Although this is fibrinogen's main job, it also forms bridges between platelets. Fibrinogen is a protein made by the liver. It is a dimer (made up of two similar units). Each unit has an alpha, beta, and gamma chain. The alpha and beta chains have a small peptide called fibrinopeptide. 
When fibrinogen is activated to form fibrin, an enzyme called thrombin cleaves the fibrinopeptides off of the alpha and beta chains.
It is possible to examine a small sample of a person's blood to find out how much fibrinogen is in the blood. This helps medical professionals to know if too much or too little fibrinogen is the cause of a patient's medical problems.