A biological standard for measurement of blood coagulation Factor VIII activity.

Because of the instability of Factor VIII (antihaemophilic factor) in plasma in vitro, and since evidence has accumulated that the level of activity varies significantly between samples of "fresh normal plasma", it is apparent that a stable biological standard is required.Two freeze-dried preparations of Factor VIII, one plasma and one concentrate preparation, were prepared and have been assayed in a collaborative trial by 20 laboratories in 9 countries.The laboratories used their own methods and altogether 248 assays were carried out. Comparison of materials of similar type gave precise estimates with all the methods used, but when materials dissimilar in type and in Factor VIII content were compared less precise results were obtained. Sources of variation, such as differences in method, differences between operators, and day-to-day variations, were analysed.Accelerated degradation tests, carried out on the freeze-dried plasma and concentrate preparations, showed that Factor VIII activity was sufficiently stable in the concentrate for it to be used as a long-term standard but that the Factor VIII activity was less stable in the plasma preparation.