Test Details: The factor VIII assay is a blood test to measure the activity of factor VIII, which is one of the proteins in the body that helps the blood clot. Decreased factor VIII activity may be due to Hemophilia A (a bleeding disorder caused by a lack of blood clotting factor VIII), presence of a Factor VIII inhibitor (antibody), Von Willebrand disease (another type of bleeding disorder). This test is used to find the cause of decreased blood clotting. The test may also be done to see how well treatment for hemophilia A is working