Considerations on activity assay discrepancies in factor VIII and factor IX products

New modified coagulation factor VIII (FVIII) and factor IX (FIX) products have been designed to improve the treatment of individuals with hemophilia A and B by increasing the interval between dosing. Although these FVIII and FIX molecules have been structurally modified to improve the circulation time, the changes have also influenced their behavior in functional assays in comparison with traditional plasma-derived or recombinant coagulation factors. The assignment of potencies for these products can be problematic because discordance in factor activity values between the commonly used one-stage clotting and chromogenic substrate assays is often observed. Discrepancies in potency assay values also exist when different assay kits and reagents are used in the same assay type. Ideally, all FVIII and FIX products should be calibrated against the World Health Organization (WHO) International Standards (IS) because the assignment of potencies in international units (IU) helps maintain treatment tradition and meaningful references for manufacturers, patients, and clinicians. The discrepant measurements, attributed to the modified structural and functional properties of these products, are manifested in their lack of commutability with the WHO IS for FVIII or FIX. Herein, we discuss the considerations upon which an assay is chosen for potency assignment and postadministration monitoring of a new factor product, which include the validity of the assay calibrated with the IS, the meaning of the potency values in IU, standards of care for patients, clinical relevance between the assigned potency value and recovery value from clinical laboratories, and patient safety.