fibrinogen Definition Fibrinogen (Factor I) is a protein that originates in the liver. It is converted to fibrin during the blood-clotting process (coagulation). Purpose The fibrinogen test aids in the diagnosis of suspected clotting or bleeding disorders caused by fibrinogen abnormalities. Precautions This test is not recommended for patients with active bleeding, acute infection or illness, or in those patients who have received blood transfusions within four weeks. Drugs that may increase fibrinogen levels include estrogens and oral contraceptives. Drugs that may cause decreased levels include anabolic steroids, androgens, phenobarbital, urokinase, streptokinase, and valproic acid. Description In order for blood to clot, fibrinogen must be converted to fibrin by the action of an enzyme called thrombin. Fibrin molecules clump together to form long filaments, which trap blood cells to form a solid clot. The conversion of fibrinogen to fibrin is the last step of the "coagulation cascade," The factors involved in the coagulation cascade are numbered I, II, and V through XIII. Factor I is fibrinogen, while factor II (fibrinogen's immediate precursor) is called prothrombin. Most of the coagulation factors are made in the liver, which needs an adequate supply of vitamin K to manufacture the different clotting factors. When fibrinogen acts as an "acute-phase reactant," it rises sharply during tissue inflammation or injury. Preparation This test is performed with a blood sample, which can be drawn at any time of day. The patient does not have to be fasting (nothing to eat or drink). Normal results Normal reference ranges are laboratory-specific, but are usually within the following: adult: 200 mg/dL-400 mg/dL newborn: 125 mg/dL-300 mg/dL Abnormal results Spontaneous bleeding can occur with values less than 100 mg/dL. Key Terms Fibrin The last step in the coagulation process. Fibrin forms strands that add bulk to a forming blood clot to hold it in place and help "plug" an injured blood vessel wall. Platelet An irregularly shaped cell-like particle in the blood that is an important part of blood clotting. Platelets are activated when an injury causes a blood vessel to break. They change shape from round to spiny, "sticking" to the broken vessel wall and to each other to begin the clotting process. Prothrombin A type of protein called a glycoprotein that is converted to thrombin during the clotting process. Thrombin An enzyme that converts fibrinogen into strands of fibrin.