The factor VIII assay is a blood test that measures the activity of factor VIII -- one of the substances involved in blood clotting (coagulation).
Plasma factor VIII antigen
A blood sample is needed. For information on how this is done, see: Venipuncture
No special preparation is needed for this test.
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.
This test is used to find the cause of too much bleeding (decreased blood clotting), or if a family member is known to have hemophilia A. The test may also be done to see how well treatment for hemophilia A is working.
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.
Decreased levels may be due to:
Increased levels may be due to:
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight but may include:
This test is most often performed on people who have bleeding problems. The risk of excessive bleeding is slightly greater for people with bleeding problems than for people without bleeding problems.
When you bleed, the body launches 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.
Kessler C. Hemorrhagic disorders: Coagulation factor deficiencies. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier;2007:chap 180.
Ragni MV, Kessler CM, Lozier JN. Clinical aspects and therapy for hemophilia. In: Hoffman R, Benz EJ Jr., Shattil SJ, et al, eds. Hoffman Hematology: Basic Principles and Practice. 5th ed. Philadelphia, Pa: Churchill Livingstone Elsevier; 2008:chap 125.