Incidence of inhibitors in patients with severe and moderate hemophilia A treated with factor VIII concentrates.

Recent data published on the prevalence of inhibitors to factor VIII in hemophiliacs on treatment show great variations, with prevalence rates ranging from 3.6 to 14.2%. We have studied the cumulative risk of inhibitor development in a cohort of 62 patients with hemophilia A. All patients were born after 1960, were natives of the Vienna area, had a factor VIII activity of less than 5%, and were treated at least once. Using the method of Cutler and Ederer, the cumulative risk of inhibitor development was found to be 24% at the age of 25 years. Most inhibitors developed between the ages of 3 and 7 years. The current prevalence of F VIII inhibitors in the group of patients studied is 17.5%. It is concluded that prevalence data underestimate the true risk of inhibitor development.