1. Overview
Fibrinogen is a soluble plasma macromolecule that plays a central role in blood coagulation. Following vascular injury, a cascade of enzymatic reactions activates thrombin, a serine protease that converts fibrinogen into fibrin. Fibrin then forms an insoluble meshwork that stabilizes the blood clot.
Under pathological conditions, the fibrin network can entrap large numbers of erythrocytes and leukocytes, forming a thrombus that may partially or completely occlude blood vessels. Thus, fibrin and fibrinogen are essential both for physiological hemostasis and for pathological thrombosis.
2. Symptoms
Abnormal fibrinogen levels do not directly cause symptoms, but clinical manifestations arise from bleeding or thrombotic complications. Low fibrinogen levels are associated with excessive bleeding, presenting as bleeding gums, gastrointestinal bleeding, blood in urine or stool, coughing up blood, excessive bruising, frequent nosebleeds, and, in severe cases, internal bleeding such as splenic rupture.
Elevated fibrinogen levels may predispose individuals to clot formation, increasing the risk of cardiovascular events, stroke, and venous thromboembolism.
3. Causes
Fibrinogen abnormalities may be inherited or acquired. Raised fibrinogen levels commonly occur in infections, inflammatory conditions, malignancies, rheumatoid arthritis, nephrotic syndrome, pregnancy, myocardial infarction, and stroke. Fibrinogen acts as an acute-phase reactant and increases in response to systemic inflammation.
Low fibrinogen levels are caused by genetic mutations in the FGA, FGB, or FGG genes, which encode fibrinogen chains. These mutations result in quantitative or qualitative fibrinogen disorders. Afibrinogenemia, inherited in an autosomal recessive manner, is characterized by the absence of fibrinogen, while hypofibrinogenemia involves reduced levels of structurally normal fibrinogen. Dysfibrinogenemia refers to structurally abnormal fibrinogen that may lead to bleeding or thrombosis.
4. Risk Factors
Risk factors for abnormal fibrinogen levels include systemic inflammation, infection, pregnancy, chronic inflammatory disorders, malignancy, liver disease, nephrotic syndrome, and inherited genetic defects.
Severely low fibrinogen levels significantly increase the risk of hemorrhage, while markedly elevated levels create a hypercoagulable state. Fibrin degradation products and D-dimer levels are also clinically important markers indicating clot breakdown and are used to evaluate conditions such as disseminated intravascular coagulation, pulmonary embolism, and deep vein thrombosis.
5. Prevention and Clinical Management
Assessment of fibrinogen levels is crucial in patients with suspected bleeding disorders, thrombotic conditions, or systemic inflammation. Indications for testing include unexplained bleeding, excessive bruising, gastrointestinal bleeding, hematuria, hemoptysis, and evaluation of clotting abnormalities.
Blood samples are collected in trisodium citrate tubes with a recommended blood-to-anticoagulant ratio of 9:1. Samples must be properly filled, mixed, and promptly transported to the laboratory. Underfilled, overfilled, hemolyzed samples or those contaminated with heparin can produce erroneous results and should be avoided.
Fibrinogen measurement is commonly performed using the Clauss clotting method, a quantitative functional assay that measures the rate of fibrinogen-to-fibrin conversion in the presence of excess thrombin. Rapid methods for detecting fibrin degradation products use antibody-coated latex particles specific for fibrin and fibrinogen fragments.
Normal fibrinogen levels range between 200 and 400 mg/dL. Levels below 50 mg/dL carry a high risk of bleeding, while levels exceeding 700 mg/dL increase the risk of thrombosis affecting vital organs such as the heart or brain.
Clinically, elevated fibrinogen levels are associated with adverse outcomes in cardiovascular disease, stroke, cancer progression, sepsis, and inflammatory disorders. Reduced fibrinogen levels indicate severe liver dysfunction, consumptive coagulopathies such as DIC, and increased hemorrhagic risk. Monitoring fibrinogen trends provides valuable prognostic information in pregnancy complications, postoperative states, COVID-19–associated coagulopathy, and critical illness.
