Inhibitory antibodies (inhibitors) against factor VIII (FVIII) develop in 25% to 35% of previously untreated patients (PUPs) with severe hemophilia A (SHA). It is the most serious complication of classic hemophilia treatment.1-3  Most inhibitors develop during the first 50 exposure days (EDs) to FVIII, with 50% of inhibitors already present after 14 to 15 EDs.2-4  After 50 EDs, inhibitor development is rare and is reported in number of new inhibitors per 1000 person-years. A recent systematic review reported an overall inhibitor incidence of 2.06 per 1000 person-years. A definition specifically designed to separate a priori high-risk and low-risk category patients was established after the outbreaks of 2 product-specific inhibitors in the 1990s.6-8  According to this definition, previously treated patients (PTPs), considered tolerant to exogenous FVIII, are patients with >150 EDs.

Hemophilia treatment has...

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