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Blood Test: Factor VIII Activity

Medically reviewed by: Yamini Durani, MD

What Is a Blood Test?

By taking and testing a small sample of a person’s blood, doctors can check for many kinds of diseases and conditions. Blood tests help doctors check how the body’s organs are working and see if medical treatments are helpful.

To help your child get ready for a blood test, find out if they need to fast (not eat or drink) or should stop taking medicines before the test. Explain what to expect during the test. If your child is anxious about it, work together on ways to stay calm.

What Is a Factor VIII Activity Test?

Proteins called clotting factors help blood clot properly and help prevent too much bleeding. A factor VIII activity blood test lets doctors see how well a protein called factor VIII is working. 

The body's clotting factors are numbered using the Roman numerals I through XIII. They work together in a special order, almost like pieces of a puzzle. When the last piece is in place, the clot develops — but if even one piece is missing or defective, the puzzle can't come together.

Factor VIII, with factor IX, is involved in the creation of a "net" that closes a torn blood vessel. An abnormal gene can make some kids not have enough factor VIII, causing the bleeding disorder known as hemophilia A. Not having enough factor IX is known as hemophilia B.

Why Are Factor VIII Activity Tests Done?

Doctors order the factor VIII activity test to help diagnose or monitor the treatment of hemophilia A. The test also can help find the reason for an abnormal result on other clotting tests, such as prothrombin time (PT) or partial thromboplastin time (PTT).

It also may be done as part of an evaluation for a bleeding disorder called von Willebrand disease. Factor VIII circulates in the body attached to another clotting factor called von Willebrand factor (vWF). So a lower amount of vWF can also mean a decreased amount of factor VIII.

What if I Have Questions?

If you have questions about the factor VIII activity test or what the test results mean, talk to your doctor.

Medically reviewed by: Yamini Durani, MD
Date reviewed: August 2023