- FVIII inhibitor positive (level of ≥0.6 Bethesda Units [BU] per mL using the Nijmegen modification of the Bethesda assay). Inhibitor status to be documented as negative prior to study enrollment according to the two most recent, consecutive inhibitor assays on record. If patients have < 50 exposure days, an assay will be completed centrally within a reasonable timeframe (approximately 8 weeks suggested) to make sure that they are negative.