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Factor VIII assay

Definition

Factor VIII assay is a blood test that looks at how a protein called factor VIII works. This protein helps blood clot.

Alternative Names

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

You doctor may order this test if you have signs of hemophilia or decreased blood clotting. The test may also be done to see how well treatment for hemophilia is working.

Normal Results

Normal results vary between labs, but in general, a result between 50% and 200% is considered normal.

What Abnormal Results Mean

Decreased levels may be due to:

• Hemophilia A
• Disseminated intravascular coagulation
• Presence of a Factor VIII inhibitor (antibody)

Additional conditions under which the test may be performed:

• Von Willebrand's disease

Risks

• Excessive bleeding
• Fainting or feeling lightheaded
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

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.

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.

Considerations

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.

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