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Potency and mass of factor VIII in FVIII products

Summary.  Several factor (F) VIII products of different origin and structure are being used for haemophilia A treatment worldwide. The assessment of FVIII concentration in these products is done using activity assays, which are dependent upon the assay and its modifications. To evaluate FVIII products for potency and for FVIII concentration and specific activity, three activity‐based assays [activated partial thromboplastin time (APTT), intrinsic FXase and synthetic coagulation proteome] and two immunoassays (ELISA and western blotting) were used in this study with albumin‐free full‐length recombinant (r) FVIII as a standard. In all activity assays, products A and B (both contain full‐length rFVIII) at 1 U mL−1 showed potency similar to that of the 0.7 nm (1 U mL−1) rFVIII standard. Product E (contains truncated rFVIII) was less potent in the APTT (83% of standard) and product C (contains plasma FVIII) was less potent in FXase assays (66%). The ELISA immunoassay revealed that the specific activity of FVIII proteins in products A–C and E varied over a wide range (3900–13 200 U mg−1) and was higher for most lots when compared with the standard (5000 U mg−1), whereas the specific activity of product D (contains plasma FVIII) was lower than expected (3200–4800 U mg−1). (i) FVIII potency estimated in different assays gives dissimilar results; (ii) the specific activity of FVIII in various FVIII products is different and inconsistent. Thus, the administration of an equal FVIII potency in units means the administration of different amounts of FVIII protein, which may partly explain apparent discrepancies in product performance.