Evaluation of factor VIII deficient plasmas.

Immunodepleted plasmas from Organon Teknika, Dade, Stago, Diagen and the Scottish National Blood Transfusion Service (SNBTS), and haemophilic plasma from Immuno were compared by several laboratories with haemophilic plasma from local donors as substrates in one-stage factor VIII assays. Five clinical plasma samples and four concentrates were assayed against British Standard plasma and International Standard concentrate. Potencies of all plasma samples were not significantly different from those with local haemophilic plasma for Immuno, Organon Teknika, Stago and SNBTS substrates. Dade differed from haemophilic on one sample and Diagen on three. Buffer blank times and slopes of standard lines were similar with all substrates. A positive drift between the beginning and end of the assay was found with the Immuno substrate and a negative drift with the Organon Teknika substrate. In the concentrate study results for all substrate plasmas were not significantly different from haemophilic on the intermediate purity and conventional high purity products. On the monoclonal and recombinant products, there was a tendency for the immunodepleted substrates to give lower potencies than the haemophilic, and significant differences were found with Dade and Stago on the monoclonal concentrate, and with Dade, Stago and Diagen on the recombinant concentrate. Overall, this study indicates that most commercially available substrate plasmas are suitable as replacements for locally collected haemophilic plasma in one-stage assays of clinical samples, and of intermediate purity and conventional high purity concentrates. For assays of very high purity concentrates (monoclonal and recombinant), haemophilic plasma is preferable as some immunodepleted plasmas give low results.

The behaviour of different factor VIII concentrates in a chromogenic factor X-activating system.

A chromogenic factor Xa generation method has been developed for comparing the co-factor activity of factor VIII concentrates at physiological factor VIII concentrations (1 iu/ml). In the presence of thrombin all concentrates gave similar rapid rates of factor Xa generation, but in the absence of thrombin there were major differences between the rates of Xa generation between different products. High purity products, particularly those prepared by monoclonal antibody purification from plasma and recombinant sources, gave more rapid Xa generation than most intermediate-purity products. There was a very strong correlation between the rate of Xa generation and the difference in factor VIII potency by one-stage and two-stage assays. These results suggest the possible presence of small amounts of activated factor VIII in some concentrates, but differences in von Willebrand factor content could also contribute towards the different rates of factor Xa generation observed.