An international collaborative study establishing a reference preparation for Antithrombin III

An international collaborative study involving 12 laboratories in 7 countries was carried out to establish a suitable reference preparation of antithrombin III (At III). The amount of At III present in two purified preparations, a freeze-dried normal plasma and local normal plasma pools was measured by clotting, immunological, and amidolytic assays. 120 assays were submitted of which 105 were accepted as valid for inclusion in subsequent analyses. Less laboratory to laboratory variation was found when At III was assayed in freeze-dried normal plasma, as compared to purified preparations of At III, and there was also less method to method variation when At III was measured in freeze-dried plasma. When measured as heparin co-factor activity, the two purified preparations contained only about half the level of At III found by immunoassay or progressive At III clotting assays. In contrast, the use of freeze-dried plasma provided results which showed excellent agreement between the various laboratories by all assays; accordingly, this material has been established by the World Health Organization as the International Reference Preparation for At III, with an assigned potency of 0.9 i.u. per ml.