Fibrinogen Antigen
Fibrinogen is a plasma protein synthesized by the liver that is cleaved by thrombin to form fibrin. The circulating half-life of plasma fibrinogen is 3 to 5 days. Fibrinogen is an acute phase reactant and may be significantly increased in acute or chronic inflammatory disorders, pregnancy and estrogen therapy. Chronically elevated fibrinogen is a risk factor for arterial thromboembolism.
Fibrinogen levels are decreased in many different medical disorders including disseminated intravascular coagulation, fibrinolysis, advanced liver disease, L-asparaginase therapy, and therapy with fibrinolytic agents such as streptokinase, urokinase and tissue plasminogen activator. Congenital fibrinogen deficiency is extremely rare and is inherited as an autosomal recessive trait. A patient may have either afibrinogenemia or hypofibrinogenemia.
Fibrinogen function abnormalities are called dysfibrinogenemias. They may be inherited or acquired. Acquired dysfibrinogenemias occur in association with chronic hepatitis, hepatoma, chronic glomerulonephritis, and hypernephroma. Patients with dysfibrinogenemia are generally asymptomatic. Congenital dysfibrinogenemias are usually inherited as autosomal dominant traits and are more likely to be associated with bleeding or thrombosis.
Patients with dysfibrinogenemias often have low fibrinogen activity. Fibrinogen antigen data should be compared with functional fibrinogen activity on the same sample for evaluation of afibrinogenemia, hypofibrinogenemia and dysfibrinogenemia.
The primary screening test for dysfibrinogenemia is a thrombin time, which is usually prolonged. The diagnosis of dysfibrinogenemia is confirmed by demonstrating low fibrinogen activity to fibrinogen antigen ratio. Functional fibrinogen activity is measures the rate of clot formation after addition of high concentrations of thrombin to patient's plasma.
Patients with dysfibrinogenemia have low fibrinogen activity and normal fibrinogen antigen level, giving a decreased ratio Patients with hypofibrinogenemia will have decreases of both fibrinogen activity and antigen, giving a normal ratio. Fibrinogen antigen is measured using radial immunodiffusion. Patient plasma is added to agar containing anti-fibrinogen antibody. The diameter of the zone of precipitation is directly proportional to fibrinogen concentration.
Specimen requirement is a light blue top (citrate) tube of blood. Reference range for fibrinogen antigen is 239 to 509 mg/dL. Reference range for fibrinogen activity to antigen ratio is 0.8 to 1.7.
