URL of this page: //www.nlm.nih.gov/medlineplus/ency/article/003678.htm

Factor VIII assay

The factor VIII assay is a blood test to measure the activity of factor VIII. This is one of the proteins in the body that helps the blood clot.

How the Test is Performed

A blood sample is needed.

How to Prepare for the Test

No special preparation is needed.

How the Test will Feel

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or slight bruising. This soon goes away.

Why the Test is Performed

This test is used to find the cause of too much bleeding (decreased blood clotting). Or, it may be ordered 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.

Normal Results

A normal value is 50 to 200% of the laboratory control or reference value.

Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or may test different samples. Talk to your doctor about the meaning of your specific test results.

What Abnormal Results Mean

Decreased factor VIII activity may be due to:

Increased activity may be due to:

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


Veins and arteries vary in size so it may be harder to take a blood sample from one person than another.

Other slight risks from having blood drawn 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 done on people who have bleeding problems. The risk of too much bleeding is slightly greater for people with bleeding problems than others.

Alternative Names

Plasma factor VIII antigen


Carcao M, Moorehead P, Lillicrap D. Hemophilia A and B. In: Hoffman R, Benz EJ Jr, Silberstein LE, Heslop HE, Weitz JI, Anastasi, J, eds.Hematology: Basic Principles and Practice

Schmaier AH. Laboratory evaluation of hemostatic and thrombotic disorders. In: Hoffman R, Benz EJ Jr, Silberstein LE, Heslop HE, Weitz JI, Anastasi J, eds.Hematology: Basic Principles and Practice

Update Date 1/27/2015

Related MedlinePlus Health Topics