Emicizumab is a monoclonal bispecific antibody with a terminal half-life of 28 days which is now licensed in the treatment of severe haemophilia A with or without inhibitors. Some heterogeneity in residual emicizumab concentrations have been reported according to age, body mass index or drug therapeutic regimen. Some cases of neutralizing antidrug antibodies have been also reported. Whether monitoring emicizumab plasma concentration could predict the residual bleeding risk under emicizumab is unknown. As conventional coagulation assays are not adapted for emicizumab monitoring, this study aims to assess the value of monitoring residual emicizumab plasma concentration by UPLC-MS/MS in bleeding risk prediction.

Inclusion Criteria:

• Adult or child with Clinical diagnosis of severe hemophilia A (FVIII activity < 1%) with or without inhibitor
• Clinical indication to emicizumab therapy

Exclusion Criteria:

• Refusal to give informed consent
• acquired hemophilia A
• other inherited or acquired bleeding disorder
• bodyweight < 10 kgs

100 patients with severe hemophilia A will be included in the study on 2 french Hemophilia Treatement Centers (CHU Lille and CHU Caen)