Overview
The Factor VIII Test measures the level or activity of Factor VIII, a key blood-clotting protein required for normal coagulation. Factor VIII works closely with other clotting factors to help form a stable blood clot and stop bleeding after an injury.
This test is mainly used to diagnose Hemophilia A, a genetic disorder caused by Factor VIII deficiency, and to evaluate unexplained bleeding or abnormal clotting results such as prolonged aPTT.
Understanding Factor VIII levels is essential for properly diagnosing bleeding disorders, monitoring treatment, managing clotting abnormalities, and guiding therapy in patients who have either too little or too much Factor VIII in their blood.
What is the Factor VIII Test?
The Factor VIII Test evaluates either:
- Factor VIII activity (how well it works), or
- Factor VIII antigen (how much is present)
This test is commonly ordered in the following situations:
- Unexplained bleeding or bruising
- Suspected Hemophilia A
- Abnormal coagulation test results (especially prolonged aPTT)
- Monitoring Factor VIII replacement therapy
- Investigating clotting tendency when levels are high
This test plays a central role in diagnosing both bleeding and clotting disorders.
Where is Factor VIII Produced in the Body?
Factor VIII is produced primarily by endothelial cells, which line the inner walls of blood vessels.
Production sites include:
1. Endothelial cells throughout the body
These cells synthesize Factor VIII and release it into the bloodstream.
2. Liver sinusoidal endothelial cells
These specialized liver cells also contribute significantly to Factor VIII production.
3. Stabilized by von Willebrand factor (vWF)
In the blood, Factor VIII circulates bound to von Willebrand factor, which protects it from breakdown and helps deliver it to the clotting site.
Unlike many other clotting factors, Factor VIII is not produced by liver hepatocytes (the main liver cells), but by endothelial cells.
Functions and Importance of Factor VIII
Factor VIII is essential for normal blood clotting. Its functions include:
1. Key role in the intrinsic pathway of clotting
Factor VIII activates Factor X, a necessary step in producing thrombin, which helps form fibrin clots.
2. Helps form stable blood clots
It reinforces the clotting cascade to ensure bleeding stops properly.
3. Identifies bleeding disorders
Measuring Factor VIII levels helps diagnose:
- Hemophilia A
- von Willebrand disease
- Acquired clotting factor deficiencies
4. Monitors treatment
Patients with Hemophilia A often receive Factor VIII replacement therapy. The test ensures correct dosage and response.
5. Assists in evaluating abnormal bleeding
The test is part of a larger evaluation when patients have:
- Prolonged aPTT
- Easy bruising
- Uncontrolled bleeding after surgery or injury
Causes of Low Factor VIII Levels
Low levels indicate problems in Factor VIII production or increased destruction.
Common causes include:
1. Hemophilia A
A genetic disorder where the body does not produce enough Factor VIII.
2. Acquired Hemophilia
The immune system mistakenly forms antibodies against Factor VIII.
3. Severe Liver Disease
Liver damage affects endothelial function, reducing Factor VIII production.
4. Disseminated Intravascular Coagulation (DIC)
Clotting factors get consumed rapidly during severe illness.
5. Massive Blood Loss or Dilution
Large transfusions may dilute Factor VIII levels.
Symptoms of Low Factor VIII Levels
When Factor VIII is low, patients experience symptoms related to poor clotting, such as:
- Easy or unexplained bruising
- Prolonged bleeding from minor cuts
- Nosebleeds or gum bleeding
- Heavy bleeding after surgery or dental work
- Bleeding into joints (hemarthrosis) causing pain and swelling
- Muscle bleeding
- Severe internal bleeding in critical cases
These symptoms are typical in Hemophilia A.
Causes of High Factor VIII Levels
Elevated Factor VIII levels increase the risk of abnormal clotting.
Common causes include:
1. Inflammation
Factor VIII is an acute-phase reactant, increasing during illness.
2. Pregnancy
Levels naturally rise during pregnancy.
3. Stress and Exercise
Temporary increases occur due to hormonal changes.
4. Liver Disease
Some liver conditions raise Factor VIII.
5. Thrombotic Disorders
Patients with DVT or pulmonary embolism often have elevated levels.
6. Certain Cancers
Malignancies may increase clotting factor production.
Symptoms of High Factor VIII Levels
High levels do not cause direct symptoms but increase blood clot risk.
Symptoms may include:
- Swelling and pain in legs (DVT)
- Chest pain and sudden breathlessness (pulmonary embolism)
- Sudden weakness or numbness (stroke)
- Chest pressure or discomfort (heart attack)
These require immediate medical attention.
Reference Ranges
General reference values:
Normal Factor VIII Activity
50% – 150% of normal
Hemophilia A Severity Levels
- Mild: 5% – 40%
- Moderate: 1% – 5%
- Severe: <1%
Ranges may vary by laboratory and assay method.
Sample Type
- Blood sample
- Collected in a citrate (blue-top) tube
- Plasma is used for testing
The sample must be processed quickly because Factor VIII is unstable.
Test Preparation
- No fasting required
- Avoid strenuous exercise before the test
- Inform your doctor if you:
- Take anticoagulants (warfarin, heparin)
- Are pregnant
- Recently had surgery or injury
- Have a family history of bleeding disorders
- Stress and illness may affect results
When to Consult a Doctor
Seek medical advice if:
Bleeding-related issues
- Frequent nosebleeds
- Excessive bruising
- Prolonged bleeding after injuries
- Joint swelling and pain
- Heavy bleeding after dental procedures
Clotting-related issues
- Swollen, painful leg (possible DVT)
- Sudden breathlessness
- Chest pain
- Sudden difficulty speaking or weakness
Lab abnormalities
- Prolonged aPTT
- Low or high Factor VIII on report
Urgent care is required if symptoms suggest internal bleeding or thrombosis.
Important Word Explanations
- Factor VIII: A protein required for blood clotting
- Hemophilia A: Genetic disorder causing Factor VIII deficiency
- aPTT: A blood test that measures clotting time
- Thrombin: Enzyme needed to form a blood clot
- DVT: Deep vein thrombosis (blood clot in the leg)
- vWF: Von Willebrand factor, stabilizes Factor VIII
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