Overview
Fibrinogen, also known as Factor I, is an essential blood-clotting protein made by the liver. It plays a central role in forming blood clots that stop bleeding. During clot formation, fibrinogen is converted into fibrin, which forms a strong mesh-like structure that stabilizes the clot at the site of injury. The Fibrinogen Test measures either the level (quantity) or the activity/function (how well it works) of fibrinogen in the blood.
Doctors commonly order this test in patients with unexplained bleeding, clotting issues, liver disease, inflammation, suspected DIC, or recurrent miscarriages. It is an important test for evaluating both bleeding disorders and thrombotic tendencies.
What is Fibrinogen (Factor I) Test?
The fibrinogen test measures:
1. Fibrinogen Level (Antigen Test)
Tells how much fibrinogen is present in the blood.
2. Fibrinogen Activity (Functional Test)
Determines how well fibrinogen converts into fibrin during clotting.
This test helps doctors determine whether abnormal bleeding, delayed clotting, or excessive clot formation is due to problems related to fibrinogen.
It is also a crucial component in evaluating patients for:
- Bleeding disorders
- Suspected DIC
- Chronic inflammation
- Severe liver disease
- Pregnancy complications
- Cardiovascular risk profiling
Where is Fibrinogen Produced in the Body?
Fibrinogen is synthesized in the liver and released into the bloodstream as a soluble protein. When bleeding occurs, it is converted into fibrin with the help of thrombin.
Because fibrinogen depends entirely on liver function, any liver disease or damage can significantly affect its production.
Main Functions and Importance of Fibrinogen
Fibrinogen performs multiple important roles in the body:
1. Forms the Basis of a Stable Blood Clot
Fibrinogen is converted into fibrin, creating the structural mesh of a clot.
2. Stops Bleeding
It is essential for preventing blood loss after injuries, dental procedures, childbirth, or surgery.
3. Helps in Wound Healing
Fibrin supports tissue repair and healing after clot formation.
4. Acts as an Acute Phase Reactant
Levels rise during:
- Infections
- Inflammation
- Trauma
- Cancers
This makes fibrinogen useful for evaluating inflammation in the body.
5. Helps Platelets Stick Together
Fibrinogen supports platelet aggregation, strengthening the clot.
Causes of Low Fibrinogen Levels
Low levels indicate that blood cannot clot properly, leading to dangerous bleeding.
1. Inherited Fibrinogen Disorders
Rare conditions include:
- Afibrinogenemia – complete absence of fibrinogen
- Hypofibrinogenemia – low fibrinogen levels
- Dysfibrinogenemia – normal amount but poor function
These are genetic and often present early in life.
2. Severe Liver Disease
Examples include:
- Cirrhosis
- Liver failure
- Chronic hepatitis
Since fibrinogen is made in the liver, damage reduces its production.
3. Disseminated Intravascular Coagulation (DIC)
A life-threatening condition where clotting happens throughout the body, consuming fibrinogen rapidly.
4. Massive Bleeding or Transfusions
Dilutes clotting factors and lowers fibrinogen levels.
5. Certain Cancers or Medications
Some cancers and drugs interfere with clotting factor production.
Symptoms of Low Fibrinogen Levels
Low fibrinogen causes bleeding symptoms, which may include:
- Excessive or prolonged bleeding after injury or surgery
- Frequent nosebleeds
- Gum bleeding
- Easy bruising
- Heavy menstrual bleeding
- Blood in urine or stool
- Recurrent miscarriages
- Rarely, internal bleeding in severe cases
Inherited deficiencies may cause severe bleeding early in life.
Causes of High Fibrinogen Levels
High fibrinogen levels are often related to inflammation and increase the risk of clots.
Common causes include:
- Acute or chronic inflammation
- Infections
- Pregnancy
- Oral contraceptives
- Estrogen therapy
- Obesity
- Smoking
- Diabetes
- Heart disease
- Certain cancers
Since fibrinogen is an acute-phase reactant, levels rise whenever the body has inflammation or tissue injury.
Symptoms of High Fibrinogen Levels
High fibrinogen itself does not cause symptoms, but increases the risk of:
- Deep Vein Thrombosis (DVT) – leg pain, redness, swelling
- Pulmonary Embolism (PE) – chest pain, breathlessness
- Stroke
- Heart attack
Elevated levels are especially dangerous in people with cardiovascular disease or metabolic disorders.
Reference Ranges
Typical normal fibrinogen levels:
200 – 400 mg/dL
Ranges may slightly differ by laboratory.
- Below 100 mg/dL increases bleeding risk
- Above 400 mg/dL increases clotting risk
Sample Type
The test uses:
- Venous blood sample
- Collected in a Light Blue Top Tube
- Additive: Sodium Citrate (prevents clotting in the tube)
Citrate binds calcium to keep the sample unclotted so fibrinogen can be accurately measured in the lab.
Test Preparation
- No fasting required
- Avoid strenuous exercise before the test
- Inform your doctor if you take:
- Blood thinners
- Hormonal medications
- Anti-inflammatory drugs
- Notify your doctor if you have:
- Liver disease
- Pregnancy
- Recent infections
- DIC symptoms (bleeding + clotting together)
When to Consult a Doctor
Seek medical help if you experience:
Bleeding Symptoms
- Frequent nosebleeds
- Heavy menstrual bleeding
- Bleeding after minor injuries
- Blood in stool or urine
- Prolonged bleeding after surgery or dental procedures
Signs of Possible Clots
- Sudden leg swelling or pain
- Sharp chest pain
- Difficulty breathing
- Sudden weakness or numbness
- Vision changes
- Severe headache
These require urgent medical attention.
Important Word Explanations
- Fibrinogen (Factor I): Protein converted into fibrin for clot formation
- Fibrin: Mesh-like structure forming a stable clot
- Acute-phase reactant: Protein that rises during inflammation
- DIC: Serious condition where clotting factors are rapidly used up
- Sodium Citrate Tube: Prevents the blood sample from clotting
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