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The factor VIII assay is a blood test that measures the activity of factor VIII -- one of the substances involved in blood clotting (coagulation).

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The area is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

No special preparation is necessary 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.

Your doctor may order this test if you have signs of easy bruising or bleeding, such as hemophilia. The test may also be done to see how well treatment for hemophilia 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.

Decreased levels may be due to:

• Disseminated intravascular coagulation
• Hemophilia A
• Presence of a Factor VIII inhibitor (antibody)
• Von Willebrand's disease

Increased levels may be due to:

• Advanced age
• Diabetes
• Liver disease
• Inflammation
• Pregnancy
• Obesity

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:

• 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 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.

Plasma factor VIII antigen

Kessler C. Hemorrhagic disorders: Coagulation factor deficiencies. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier;2007:chap 180.

Updated by: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; and Yi-Bin Chen, MD, Leukemia/Bone Marrow Transplant Program, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.