Hemophilia A and B are X-linked inherited bleeding disorders characterized by decreased factor VIII and IX, respectively. Hemophilia A is the more common disorder, affecting approximately 82% of all patients with hemophilia.1 The severity of hemophilia A is defined by the level of factor VIII activity. Severe, moderate, and mild hemophilia are associated with factor VIII activity levels of <1%, 1% to 5%, and >5% to <40%, respectively.2 Patients with severe or moderate hemophilia A often need specific substitution therapy with plasma-derived or recombinant factor VIII concentrates. A major complication of replacement therapy is the development of alloantibodies (inhibitors) that inactivate factor VIII.
Autoantibodies may also develop in patients without a congenital disorder (referred to as acquired hemophilia), resulting in life-threatening bleeding symptoms.3 Acquired hemophilia can develop in patients with immunologic disorders such as rheumatoid arthritis, post-partum women, and in older individuals with no underlying disease.