OPTIMS is a non interventional validation study of the calculator developed by Bayer for clinician's use in the prophylactic treatment by factor VIII of patients with severe or moderate Haemophilia A with a severe clinical profile.

The study takes place during a single visit, at the time of patient enrollment in the study

Inclusion Criteria:

• Patient with severe haemophilia A defined as residual factor VIII activity < 1%
• or Patient with moderate haemophilia A (residual factor VIII activity >1% and <5%) with severe clinical profile
• Patient treated in prevention with the same plasma or recombinant factor VIII for at least 6 months
• Patient whose inclusion visit is performed during a routine visit including a measurement of plasma residual factor VIII :c level
• In the medical file (retrospective data) the pharmacokinetic parameters are available and have been obtained from a PK analysis of factor VIII level performed with the same anti haemophilic factor than the one used on the inclusion day

Exclusion Criteria:

• Patients with haemophilia B

• Patient with severe haemophilia A defined as residual factor VIII activity < 1%
• or Patient with moderate haemophilia A (residual factor VIII activity >1% and <5%) with clinical signs of frequent, severe bleeding episodes
• Patient treated prophylatically with the same plasma or recombinant factor VIII for at least 6 months