Introduction and Clinical Significance

Cardiovascular disease leads to 17.9 million deaths every year in the world as a result of being the main cause of death globally according to the World Health Organization.  Among the biomarkers that have been studied, plasma fibrinogen has shown promise as a marker of cardiovascular risk as it reflects both thrombotic potential and the inflammatory processes that occur in the body.

Fibrinogen is a liver produced glycoprotein which acts as precursor to fibrin the major structural component of blood clots. Apart from its basic role in hemostasis, fibrinogen serves as an acute phase reactant because its concentration increases in inflammatory states. The dual role of fibrinogen in cardiovascular risk stratification is due to its reflection of both the prothrombotic state and the chronic low grade inflammation that occurs in atherosclerotic disease progression.

Highly sensitive and specific ELISA kits for fibrinogen have improved the ability of clinicians to measure this biomarker in clinical settings.

Fibrinogen Structure and Physiological Functions

Fibrinogen is a large protein made up of three polypeptide chains namely Aα, Bβ, and γ with a molecular weight of approximately 340 kDa. The protein structure is made of a central E domain connected to two outer D domains by coiled-coil regions to form a characteristic dumbbell-shaped molecule.

The normal physiological level of fibrinogen in plasma of healthy adults is 200 to 400 mg/dL. The half life of the protein is about 3-4 days and its synthesis is mainly controlled by inflammatory cytokines such as interleukin 6 (IL 6), tumour necrosis factor alpha (TNF α) and interleukin 1 beta (IL 1β). The cytokine regulated mechanism is responsible for the increase of fibrinogen levels by 2-5 times during acute inflammatory responses.

Fibrinogen has other roles in the body besides its role in hemostasis. The protein serves as a ligand for various cell surface receptors, such as integrin αIIbβ3 on platelets and αMβ2 on leukocytes, and thus facilitates cell adhesion and migration processes. In addition, fibrinogen affects plasma viscosity and red blood cell aggregation which directly affects cardiovascular hemodynamics and tissue perfusion.

ELISA-Based Fibrinogen Quantification Methods

Present day ELISA based fibrinogen quantification employs advanced immunoassay platforms that are more sensitive and specific than traditional functional assays. These immunoassays make use of monoclonal or polyclonal antibodies that have been designed to recognize fibrinogen epitopes, thus allowing for the detection of fibrinogen even in the presence of fibrin degradation products or other interfering substances.

The ELISA assay for fibrinogen measurement uses a sandwich immunoassay protocol where capture antibodies fixed on microplate wells bind fibrinogen molecules present in plasma samples. The detection antibodies that have been conjugated with enzymes like horseradish peroxidase or alkaline phosphatase are added after washing and incubation to form the sandwich complex. A colorimetric signal that is proportional to fibrinogen concentration is produced by the addition of substrate which is then quantified using spectrophotometric analysis.

It is crucial to have quality control measures for reliable fibrinogen ELISA results, including certified reference materials, appropriate calibration curves and internal quality controls. The coefficient of variation for well-validated fibrinogen ELISA kits is usually between 3-8% for intra assay precision and 5-12% for inter assay precision. The performance characteristics of ELISA based fibrinogen measurement make it appropriate for both clinical decision making and large scale epidemiological studies.

Fibrinogen as a Cardiovascular Risk Predictor

Extensive epidemiological evidence proves that elevated plasma fibrinogen levels act as a solitary risk factor which increases the chance of cardiovascular events. Research data from prospective cohort studies show that people with fibrinogen levels in the highest tertile or quartile have a 1.5 to 2.5 times higher risk of coronary heart disease, stroke and peripheral arterial disease than people with lower fibrinogen levels.

Elevated fibrinogen levels increase blood thickness and promote platelet aggregation which leads to a prothrombotic state that results in arterial occlusions. Fibrinogen plays an active role in forming atherosclerotic plaques through its interaction with endothelial and smooth muscle cells which leads to inflammatory cell recruitment and extracellular matrix production.

Clinical Applications in Risk Stratification

The addition of fibrinogen levels to established risk assessment tools including the Framingham Risk Score or Systematic Coronary Risk Evaluation (SCORE) allows reclassification of patients with intermediate risk to high risk thus affecting their treatment options.

People with normal health status who have elevated fibrinogen levels should receive closer monitoring and possible treatment because these levels indicate inflammatory processes and increased thrombotic risk.

Fibrinogen measurement in clinical settings is most beneficial when used to evaluate risk in three particular patient groups: the elderly, patients with diabetes mellitus, and those with chronic inflammatory diseases. These groups typically present with elevated baseline fibrinogen levels and serial measurements help track disease progression and treatment response during the course of time.

Population-Based Reference Values and Variations

Multiple large epidemiological studies have demonstrated that fibrinogen levels vary substantially based on demographic characteristics and lifestyle elements and genetic polymorphisms.

Age stands as the major determinant which influences fibrinogen levels because concentrations rise by 10-15 mg/dL throughout every ten years of life. The natural aging process leads to rising fibrinogen levels because of sustained inflammation and declining hepatic production efficiency and enhanced oxidative stress. The levels of fibrinogen differ between genders and women tend to have higher levels than men especially before menopause and after menopause.

Different ethnic groups exhibit different fibrinogen concentration levels with African Americans having the highest levels compared to Caucasians and Asians. The observed differences in cardiovascular disease occurrence rates between ethnic groups may be influenced by these variations in fibrinogen concentrations.

Future Directions and Clinical Implementation

The future of fibrinogen-based cardiovascular risk assessment will depend on developing advanced personalized systems which combine several biomarkers with clinical data. Point-of-care testing devices which measure fibrinogen are under development to enable fast risk evaluation in emergency departments and outpatient clinics.

The predictive power for cardiovascular risk can be enhanced through the combination of fibrinogen with inflammatory biomarkers including high-sensitivity C-reactive protein, interleukin-6 and soluble CD40 ligand.

The clinical adoption of fibrinogen testing needs thorough evaluation of its cost-benefit ratio alongside suitable laboratory infrastructure and proper physician training. Major cardiovascular societies work to develop guidelines for fibrinogen-based risk assessment to standardize measurement protocols and interpretation criteria among different healthcare systems.