Fibrinogen, and fibrin degradation products
Fibrinogen is a circulating plasma protein manufactured by the liver. Thrombin converts fibrinogen to fibrin in the final stage of blood coagulation. Low fibrinogen levels can occur as a result of severe liver disease or due to a disorder such as disseminated intravascular coagulation (DIC). Fibrinogen is quantified by adding thrombin to a series of successively more dilute plasma samples and comparing clotting time to a control series. A coagulation analyzer is used to determine clotting time which will be inversely proportional to the concentration of fibrinogen. Normal values are approximately 200-400 mg/dl. Fibrinogen can also be measured directly by immunoassay. In this test a fibrinogen antibody is added to a plasma sample and then fibrinogen marked with the antibody is measured.
Fibrin degradation products (FDP), also known as fibrin slpit products, are present in blood when the thrombolytic enzyme plasmin cleaves fibrin or fibrinogen. Plasmin is produced when the thrombolytic system is activated. D-dimers are an FDP produced when fibrin is cleaved by plasmin. The presence of D-dimers or FDP may be used to assist with the diagnosis of DIC, Deep Venous Thrombosis (DVT) or Pulmonary Embolism (PE). However, because they are produced under a variety of circumstances their presence alone is not diagnostic.