The hemophilias are a group of related bleeding disorders that most commonly are inherited. Inherited bleeding disorders include abnormalities of coagulation factors and platelet function; the most common of these disorders is von Willebrand disease. However, when the term "hemophilia" is used, it most often refers to the following two disorders:
●Factor VIII deficiency (hemophilia A)
●Factor IX deficiency (hemophilia B, also called Christmas disease)
Patients with hemophilia, particularly those with severe disease, develop bleeding episodes that are treated with replacement of the missing factor (ie, factor VIII or factor IX concentrates). A complication of hemophilia treatment is the development of an inhibitor, which usually occurs shortly after replacement therapy has been initiated. The inhibitors are antibodies (primarily IgG) directed against the specific deficient factor.
Development of inhibitors is typically assessed in relationship to the number of exposure days (ie, days on which the patient has received one or more doses of replacement factor).