Thrombogenicity of factor IX concentrates: in vitro and in vivo (rabbit) studies

The reproducibility and correlation between the NAPTT, TGt50 in (vitro) tests and the Wessler (rabbit) stasis thrombus (in vivo) model have been studied using 10 different factor IX concentrates. The TGt50 test was more reproducible than the NAPTT and the overall reproducibility of the rabbit model was poor. The low reproducibility of the rabbit model appeared to be largely confined to those factor IX concentrates which showed a poor correlation between the NAPTT and TGt50 results. The TGt50 test emerged as the in vitro test which correlated most closely with the in vivo (rabbit) test. It is concluded that the NAPTT and TGt50 test are measuring different thrombogenic moieties in factor IX concentrates and that further studies are required to elucidate this phenomenon.