Haemophilic arthropathy is one of the major complications of severe haemophilia. In order to maintain plasma clotting factor activity levels above 1% and avoid spontaneous joint bleeds and other serious bleeding events, prophylactic factor replacement therapy is used. Because of the high cost and limited availability of clotting factor concentrates, dosing is a crucial issue for prophylaxis therapy. Several studies reported a better correlation between clinical bleeding tendency of patients with haemophilia and thrombin generation assay results compared to FVIII/FIX levels. However there is no specific data showing that thrombin generation may be a better indicator of the clinical efficacy of prophylaxis compared to the conventional FVIII measurement.

The main objective of this open, multicentre, prospective study is to compare trough thrombin generation capacity and FVIII levels in severe haemophilia patients and compare these two laboratory results with:

• the number of spontaneous haemarthroses and other spontaneous serious bleeds occurred in the last 6 months
• the number of additional FVIII units used in the last 6 months

This project requires no change in term of type of treatment : During the study, each patient will be treated by his usual clotting factor at the usual regimen (frequency and dosage).

The clinical outcome with the usual prophylaxis regimen will be correlated to TGA results

Inclusion Criteria:

• Severe haemophilia A (FVIII < 1 IU/dl)
• Currently on prophylactic therapy
• The same prophylaxis regimen in the last six months
• Age: 2 (with a body weight ≥12.5 kg ) - 45 years
• Adequate venous access
• Haemophilia patient's diary or equivalent regularly completed
• Ability of patient or family (for minors) to give informed consent
• Subject with health insurance

Exclusion Criteria:

• Age <2 and >45 years
• Haemophilia A with documented inhibitor in the last 12 months
• Clinically symptomatic liver disease (historical evidence documented in patient's medical file)
• Platelet count < 100x109/l
• Poor venous access
• Presence of a documented target joint
• Subject under legal protection measure.