Factor VIII assay
The factor VIII assay is a blood test that measures the activity of factor VIII. This is one of the substances involved in blood clotting (coagulation).
Plasma factor VIII antigen
How the Test is Performed
A sample of blood will be taken from your vein.
How to prepare for the test
You do not need to take special steps to prepare for this test.
How the Test will Feel
You may feel slight pain or a sting when the needle is inserted to draw blood. You may feel some throbbing afterward.
Why the Test is Performed
This test is used to find the cause of too much bleeding (decreased blood clotting). It can also be used if a family member is known to have hemophilia A. The test may be done to see how well treatment for hemophilia A is working, as well
A normal value is 50 - 200% of the laboratory control or reference value.
Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
The examples above show the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.
What Abnormal Results Mean
Decreased levels may be due to:
Increased levels may be due to:
Veins and arteries vary in size so it may be harder to take a blood sample from one person than another.
Other slight risks from having blood drawn may include:
- Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
This test is most often done on people who have bleeding problems. The risk of too much bleeding is slightly greater for people with bleeding problems than others.
When you bleed, the body starts a series of activities that help the blood clot. This is called the coagulation cascade. The process involves special proteins called coagulation factors (factor VIII is a coagulation factor).
Each factor's reaction triggers the next reaction. The final product of the coagulation cascade is the blood clot
. Blood clots may not form normally if any one of the clotting factors is abnormally low.
Carcao M, Moorehead P, Lillicrap D. Hemophelia A and B. In: Hoffman R, Benz EJ Jr, Silberstein LE, Heslop HE, Weitz JI, eds. Hematology: Basic Principles and Practice. 6th ed. Philadelphia, PA: Saunders Elsevier; 2012:chap 137.
Ragni M. Hemorrhagic disorders: Coagulation factor deficiencies. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Saunders Elsevier; 2011:chap 177.
Yi-Bin Chen, MD, Leukemia/Bone Marrow Transplant Program, Massachusetts General Hospital. Also reviewed by A.D.A.M. Health Solutions, Ebix, Inc., Editorial Team: David Zieve, MD, MHA, Bethanne Black, Stephanie Slon, and Nissi Wang.