ATHN 2: Factor Switching Study

This is a longitudinal, observational study of patients with Hemophilia A or B who are planning to switch to a newly approved coagulation factor replacement product, or who have recently switched factor products. The study will follow each patient for up to 1 year. Patients will be recruited at Hemophilia Treatment Centers (HTC) which are ATHN-affiliates. The primary outcome being studied is the development of inhibitor (i.e., antibodies to factor) at 1 year or 50 exposure days, whichever comes first. The study will be conducted at approximately 30 HTCs, with a planned enrollment of 600 patients.The entire study duration is projected to be approximately 6 years. In addition, optional substudies will be included for some products, as "Product-Specific Modules". These will be questionnaires to collect data for subjects receiving selected Factor products. For example, subjects receiving Kovaltry will be approached to participate in the 'Kovaltry Product-Specific Module'; subjects receiving Adynovate will be approached to participate in the 'Adynovate Product-Specific Module'. Questions will be related to product use, perceptions of product use, and other post-marketing consumer data.