Factor VIII assay

Definition:

Factor VIII assay is a test to measure the activity of factor VIII (one of the proteins important for coagulation) in the blood.

Alternative Names:

Plasma factor VIII antigen

How the test is performed:

Adult or child:
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill 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 tourniquet 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.

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:

There is no special preparation.

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:

The normal result is 50 to 200% of the laboratory "control" or reference value.

What abnormal results mean:

Decreased levels may indicate:

Congenital deficiency of factor VIII (that is, hemophilia).
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

What the risks are:

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

Note: 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.

Special considerations:

Coagulation (blood clotting) results from a sequence (cascade) of reactions involving the coagulation factors. Some of these factors have other names, for example, Factor I (fibrinogen), Factor II (prothrombin), and Factor XII (Hageman factor).

Most of these proteins are produced in the liver and secreted into the blood. Some of the Factors (that is, II, VII, IX, and X) require Vitamin K for their synthesis. Warfarin (Coumadin) is a commonly-used "anticoagulant" drug. It acts in the liver by inhibiting the enzyme that requires Vitamin K.

The coagulation sequence is initiated when some of the coagulation factors contact damaged tissue. Each coagulation factor reaction triggers the next reaction in the cascade. The final product of the coagulation cascade is the fibrin clot (blood clot).

Substances that inhibit the action of the coagulation factors, and plasmin, which eventually lyses (breaks down) the fibrin clot, are activated by the damaged tissue at the same time as the coagulation factors. However, they function more slowly and over a longer period of time than the coagulation factors.

This allows a clot to form to stop bleeding, then the clot is dissolved (after enough time for the tissue to heal) to restore blood flow. One major coagulation inhibitor is antithrombin III, a protein that requires endogenous (made in the body) heparin for its activity.

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.

Review Date: 10/9/2001
Reviewed By: Rebecca Elstrom, M.D., University of Pennsylvania Medical Center, Philadelphia, PA. Review provided by VeriMed Healthcare Network.

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