Plasma factor VIII antigen

Alternative names 
Factor VIII assay

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

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
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
A result between 50% and 200% of the laboratory reference value is considered normal.

What abnormal results mean
Decreased levels may indicate:

     
  • Hemophilia A  
  • 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

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 of reactions involving several proteins known as coagulation factors. Some of these factors have other names. For example, Factor I is also called fibrinogen, Factor II is prothrombin, and Factor XII is Hageman factor. The liver produces these proteins and secretes them into the blood. In addition, vitamin K is important to blood clotting because your body converts it into prothrombin.

Some people take warfarin to keep their blood from clotting. Warfarin inhibits prothrombin, thus interrupting the clotting cascade. Because of the link between vitamin K and prothrombin, people who take warfarin need to have consistent levels of vitamin K in their diet, as instructed by their doctor.

Coagulation begins when some of the coagulation factors contact damaged tissue. Each factor reaction triggers the next reaction, in a cascade. The final product of the coagulation cascade is the blood clot.

Substances that inhibit or “turn off” the action of the coagulation factors are activated along with the coagulation factors. These substances function more slowly, over a longer period of time, than the coagulation factors. This allows a clot to form, stopping the bleeding, then, after enough time has passed for the tissue to heal, the clot is dissolved and blood flow restored. The most important coagulation inhibitor is antithrombin III.

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.

Johns Hopkins patient information

Last revised: December 3, 2012
by Gevorg A. Poghosian, Ph.D.

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