Factor VIII assay
Key points
- Factor VIII assay is a blood test to measure the activity of factor VIII (one of the proteins important for coagulation).
- Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand.
- When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation.
- Plasma factor VIII is usually measured to diagnose or monitor treatment for hemophilia.
Alternative names
Plasma factor VIII antigen
Definition
Factor VIII assay is a blood test to measure the activity of factor VIII (one of the proteins important for coagulation).
How the test is performed
Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to swell with blood.
A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.
For an infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.
How to prepare for the test
No special preparation is necessary for this test.
How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed
Plasma factor VIII is usually measured to diagnose or monitor treatment for hemophilia. This test may be used to detect the specific cause of excessive bleeding (decreased blood clotting).
Normal Values
A result between 50% and 200% of the laboratory reference value is considered normal.
What abnormal results mean
Decreased levels may indicate:
- Hemophilia A
- Disseminated intravascular coagulation, a state in which the clotting factors are used up due to diffuse coagulation of the blood
- Presence of a Factor VIII inhibitor (antibody)
Additional conditions under which the test may be performed:
- Von Willebrand's disease
Written and revised by
Andrey Nailor, M.D.
Our doctors write and review articles for accuracy and keep them up to date.
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