There are large inter-individual differences in the bleeding pattern of patients with moderate or mild hemophilia. The major determinant of bleeding phenotype is the level of coagulant factor VIII or IX. In hemophilia A, studies addressing the association between factor VIII level and the clinical bleeding pattern yield conflicting results. In hemophilia B such studies have not yet been performed.

The primary aim of this project is to analyze the association between factor VIII and factor IX levels and the bleeding phenotype. The secondary aim is to analyze potential differences in phenotype between hemophilia A and B.

The project is a multicentre observational cohort study. We will include 230 patients with moderate or mild hemophilia A or B (FVIII/FIX 0.02-0.35 IU/mL) who are 12 to 55 years old. The main cohort study consists of clinical data collection, one blood sample and an online questionnaire for patients. Data will be collected on the nature and duration of all bleeding episodes, disease and treatment characteristics, physical activity level and musculoskeletal status. One blood withdrawal will be performed for centralized laboratory assays for FVIII or FIX levels (both one-stage and chromogenic assays) and genetic analysis for the most prevalent prothrombotic mutations. The online questionnaire for patients focuses on bleeds experienced in the past.

A subset of 50 patients aged 24 years or older with mild and moderate hemophilia A will be investigated in more detail by longitudinal data collection including analysis of physical joint status, MRI imaging of joints and biomarkers for joint damage. This longitudinal observation will consist of two time points that lie two years apart, allowing us to identify any changes that occur over the observed time period with respect to joint status.

Inclusion Criteria:

• Moderate or mild hemophilia A (FVIII:C 0.02-0.35 IU/mL) or hemophilia B (FIX:C 0.02-0.35 IU/mL)
• Age from 12 up to and including 55 years

Exclusion Criteria:

• Other clotting disorder
• Participation in another trial with an investigational product
• Comorbidity affecting the musculoskeletal status
• Clinically relevant inhibitor status at present or in the past
• Hemophilia B Leyden
• Use of anticoagulants

Patients will be recruited from Hemophilia Treatment Centers participating in the INSIGHT consortium, a well-established and productive group of investigators from European countries, Canada and Australia.