Fibrinogen

Overview of Fibrinogen Assay

Definition: A quantitative assay that measures the concentration of fibrinogen (Factor I) in plasma. Fibrinogen is a key protein in the coagulation cascade, essential for forming the fibrin clot
Clinical Significance:
- Diagnosis of Bleeding Disorders: Low fibrinogen levels can contribute to bleeding tendencies
- Evaluation of Liver Function: Fibrinogen is synthesized in the liver, so levels can be decreased in liver disease
- Assessment of Disseminated Intravascular Coagulation (DIC): Fibrinogen levels are often decreased in DIC due to consumption
- Diagnosis of Inherited Fibrinogen Disorders: Afibrinogenemia, hypofibrinogenemia, and dysfibrinogenemia
- Assessment of Thrombotic Risk: Fibrinogen is an acute-phase reactant, and elevated levels may be associated with an increased risk of thrombosis
Methods:
- Clauss Method (Functional Assay)
- Immunologic Methods

Principle: The Clauss method is a functional assay that measures the rate of clot formation after the addition of a high concentration of thrombin to diluted plasma. The clotting time is inversely proportional to the fibrinogen concentration
Procedure:
1. Plasma Dilution: The patient’s plasma is diluted with a buffer to a specific concentration
2. Thrombin Addition: A high concentration of thrombin is added to the diluted plasma
3. Clot Detection: The time to clot formation is measured, either manually or using an automated coagulation analyzer
4. Calculation: The fibrinogen concentration is determined by comparing the clotting time to a calibration curve generated using fibrinogen standards of known concentrations
Advantages:
- Measures the functional activity of fibrinogen, which is important for assessing the patient’s ability to form a stable clot
- Relatively simple and widely available
Limitations:
- Can be affected by inhibitors of thrombin (e.g., heparin, direct thrombin inhibitors)
- May be inaccurate in patients with dysfibrinogenemia (abnormal fibrinogen molecule)

Principle: These methods measure the amount of fibrinogen protein, regardless of its function
Methods:
- Nephelometry: Measures the turbidity caused by the formation of antigen-antibody complexes
- Turbidimetry: Measures the decrease in light transmission through a solution as antigen-antibody complexes form
Procedure:
1. Add a specific antibody that targets fibrinogen to the sample
2. Antigen-Antibody Complexes forms turbidity
3. Measures the amount of the turbidity to determine how much fibrinogen is in the sample
Advantages:
- Not affected by inhibitors of thrombin
- May be more accurate in patients with dysfibrinogenemia
Limitations:
- Does not assess the functional activity of fibrinogen
- Can be affected by interfering substances (e.g., lipemia, high protein levels)

Collection Tube: Sodium citrate (light blue top) tube with a 3.2% or 3.8% sodium citrate concentration
Blood-to-Anticoagulant Ratio: The correct ratio is critical for accurate coagulation testing:
- 9:1 ratio of blood to anticoagulant
- Underfilling the tube can lead to falsely prolonged clotting times due to excess anticoagulant
- Overfilling the tube can lead to falsely shortened clotting times due to insufficient anticoagulant
Processing:
- Centrifuge the sodium citrate tube to obtain platelet-poor plasma (PPP)
- PPP should have a platelet count < 10 x 10^9/L
- Test should be performed within 4 hours of collection or the plasma should be frozen to maintain optimal value of fibrongien

Pre-Analytical Variables:
- Improper Collection Technique: Tissue thromboplastin contamination or hemolysis
- Incorrect Blood-to-Anticoagulant Ratio: Underfilling or overfilling the collection tube
- Clotted Sample: Invalidates the results
- Delayed Testing: Fibrinogen can degrade over time
- Improper Storage: Incorrect storage temperatures can affect results
Analytical Variables:
- Instrument Malfunction: Ensure proper calibration and maintenance of the coagulation analyzer
- Reagent Problems: Use fresh, properly stored reagents and follow the manufacturer’s instructions
- Lipemia or Icterus: Can interfere with optical clot detection or turbidimetric measurements
Patient-Related Variables:
- Medications: Fibrinolytic agents (e.g., tPA, streptokinase) can decrease fibrinogen levels
- Liver Disease: Impairs the synthesis of fibrinogen
- Disseminated Intravascular Coagulation (DIC): Consumption of fibrinogen

Reference Range: Varies depending on the method and laboratory; a typical range is 200-400 mg/dL
Elevated Fibrinogen:
- Acute Phase Reaction: Fibrinogen is an acute-phase reactant, so levels can increase during inflammation, infection, trauma, or surgery
- Pregnancy: Fibrinogen levels increase during pregnancy
- Oral Contraceptive Use
- Increased Risk of Thrombosis
Decreased Fibrinogen:
- Disseminated Intravascular Coagulation (DIC): Consumption of fibrinogen
- Liver Disease: Impaired fibrinogen synthesis
- Inherited Fibrinogen Disorders:
  - Afibrinogenemia: Complete absence of fibrinogen (rare)
  - Hypofibrinogenemia: Reduced levels of fibrinogen
  - Dysfibrinogenemia: Presence of a dysfunctional fibrinogen molecule that does not clot properly
  - Amylodosis

Fibrinogen: A protein (Factor I) that is converted to fibrin, the main component of a blood clot
Clauss Method: A functional assay to measure fibrinogen concentration
Dysfibrinogenemia: Presence of a dysfunctional fibrinogen molecule
Afibrinogenemia: Complete absence of fibrinogen
Hypofibrinogenemia: Reduced levels of fibrinogen
Disseminated Intravascular Coagulation (DIC): A consumptive coagulopathy with widespread clotting and bleeding
Cryoprecipitate: A blood product that is rich in fibrinogen