Predictive parameters for spontaneous joint bleeding during emicizumab prophylaxis

Emicizumab is approved for prophylaxis of patients with hemophilia A (HA). Despite its efficacy in reducing bleeding, a few patients on emicizumab still experience hemarthrosis, but no tool is yet available to identify those at higher risk of spontaneous joint bleeding. To evaluate whether laboratory measurements (global coagulation assays and emicizumab concentration) and/or arthropathy scores can distinguish patients at higher risk of spontaneous joint bleeding while on emicizumab prophylaxis. Thrombin generation assay (TGA) was assessed upon the addition of tissue factor and synthetic phospholipids. Non-activated thromboelastography (NATEM) was performed in citrated whole blood. Emicizumab concentrations were measured with a modified one-stage FVIII assay. The degree of hemophilic arthropathy was assessed with the Haemophilia Joint Health Score (HJHS) and Hemophilia Early Arthropathy Detection with Ultrasound score (HEAD-US). A Cox proportional hazards model was used to evaluate the association between variables and bleeding. The predictive power of these variables was investigated by ROC analysis. Forty HA patients with and without inhibitors on emicizumab prophylaxis were enrolled in an observational cohort study. Ten of 40 developed spontaneous joint bleeding. None of the lab parameters were able to distinguish patients at higher risk of spontaneous joint bleeding. ROC analysis showed that during emicizumab prophylaxis only the presence of synovitis and a higher HEAD-US score were associated with spontaneous joint bleeding (AUC 0.84). A greater degree of arthropathy and the presence of synovitis could help to predict the risk of spontaneous joint bleeding in HA patients on emicizumab prophylaxis.