[Thrombin formation in factor IX concentrates and FEIBA (factor eight inhibitor bypassing activity) (proceedings)]

Having observed a marked increase of the fibrinopeptide A (FPA) level in vivo in 5 patients after the administration of factor IX concentrates, 8 factor IX concentrates (IX-K) and one FEIBA (factor eight inhibitor bypassing activity) fraction have been studied in vitro to ascertain whether thrombin was present or could be generated. Using fibrinogen as a substrate, the release of FPA under various conditions was measured by radioimmunoassay and it was found that (1) the addition of CaCl2 to the IX-K was necessary to produce FPA release; (2) the reaction was mainly dependent on the incubation time of the concentrate and the CaCl2 and (3) the release of FPA could be inhibited by heparin. The FEIBA fraction instantly produced FPA with or without the presence of CaCl2. It is therefore questioned whether the main effect of such products is due to a "factor II bypassing activity", i.e. thrombin.