Definition

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.

Alternative Names

Plasma factor VIII antigen; Antihemophilia factor; AHF

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 health care provider about the meaning of your specific test results.

What Abnormal Results Mean

Decreased factor VIII activity may be due to:

• Hemophilia A (a bleeding disorder caused by a lack of blood clotting factor VIII)
• Disorder in which the proteins that control blood clotting become overactive disseminated intravascular coagulation (DIC)
• Presence of a Factor VIII inhibitor (antibody)
• Von Willebrand disease (another type of bleeding disorder)

Increased activity may be due to:

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

Risks

There is little risk involved with having your blood taken. Veins and arteries vary in size from one person 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 lightheaded
• Multiple punctures to locate veins
• 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.

References

Carcao M, Gomez K, Davide M, Pierce GF. Hemophilia A and B. In: Hoffman R, Benz EJ, Silberstein LE, et al, eds. Hematology: Basic Principles and Practice. 8th ed. Philadelphia, PA: Elsevier; 2023:chap 134.

Ragni MV. Hemorrhagic disorders: coagulation factor deficiencies. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 165.

Sarode R, Kessler CM. Coagulation and fibrinolysis. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 40.