Persons with severe hemophilia A exhibit significant variations in clinical phenotype; approximately 10% are milder bleeders. This study explored factors that might contribute to this inter-patient heterogeneity. Using a multi-center prospective study following boys with severe hemophilia A on a tailored primary prophylaxis regimen, age at first joint bleed, body mass index, protocol adherence and family activity level were modeled on the age at escalation from once-weekly to twice-weekly and alternate day infusions. Escalation, based on bleeding frequency, served as a surrogate marker of bleeding severity. We also assessed the roles of clinical and hemostatic parameters in predicting age at first joint bleed using a cross-sectional study. Results suggest that a delayed first joint bleed indicates later escalation (milder bleeding pattern) and that a longer lag time is associated with an earlier onset of joint bleeding. Larger studies are needed to definitively identify predictors of clinical heterogeneity in these patients.

Persons with severe hemophilia A exhibit significant variations in clinical phenotype; approximately 10% are milder bleeders. This study explored factors that might contribute to this inter-patient heterogeneity. Using a multi-center prospective study following boys with severe hemophilia A on a tailored primary prophylaxis regimen, age at first joint bleed, body mass index, protocol adherence and family activity level were modeled on the age at escalation from once-weekly to twice-weekly and alternate day infusions. Escalation, based on bleeding frequency, served as a surrogate marker of bleeding severity. We also assessed the roles of clinical and hemostatic parameters in predicting age at first joint bleed using a cross-sectional study. Results suggest that a delayed first joint bleed indicates later escalation (milder bleeding pattern) and that a longer lag time is associated with an earlier onset of joint bleeding. Larger studies are needed to definitively identify predictors of clinical heterogeneity in these patients.