The Preparation of an Artificial Reagent for the One-Stage Factor VIII Assay

Importance of a quality assurance scheme for factor VIII assays in quality monitoring of human plasma destined for fractionation into factor VIII concentrate

A national quality assurance scheme has been established to monitor the validity of factor VIII assays performed by the various laboratories of the Scottish National Blood Transfusion Service engaged in collection and processing of donor plasma destined for fractionation. The results over the first 3-year period show that comparable assay values can be obtained by participating centres using a common standard, despite differences in equipment, methods or substrate chosen for the one-stage assay. The results also showed that chromogenic factor VIII assays correlated well with the one-stage method. Random factor VIII assays performed on plasma, harvested and frozen within 18 h from collection, were analysed to validate recently proposed Scottish specifications which stipulate that 70% of plasma donations destined for fractionation should contain at least 0.7 IU/ml. Plasma harvested and frozen between 8 and 18 h from collection did not meet the specified level in any of the regional centres. This nationally specified level was also not met by plasma harvested and frozen within 8 h from collection in spite of being achieved individually by three regional centres. Assays performed on large plasma pools at the Fractionation Centre suggested loss of some factor VIII during storage, transportation and thawing of plasma prior to bulk processing.

Artificial factor VIII deficient plasma: preparation using monoclonal antibodies and its use in one stage coagulation assays

Monoclonal antibodies to factor VIII antigen (VIII:Ag) and von Willebrand factor (vWf:Ag) were immobilised on Sephacryl S-1000 and tested for their ability to deplete normal human citrated plasma of factor VIII. A combination of two antibodies to VIII:Ag and one antibody to vWf:Ag was required to produce plasma containing less than 0.01 IU/ml. Its performance in the one stage coagulation assay of VIII:C was equivalent to that of congenital VIII deficient plasma for the assay of normal and haemophilic plasma and factor VIII concentrates. Storage of freeze dried aliquots of this product at -20 degrees C, +4 degrees C, and 37 degrees C showed that it could be used as a substrate for at least six months when stored at temperatures +4 degrees C and below.

Preparation of factor VIII-deficient plasma by immunoadsorption

An immunoadsorbent was prepared by coupling rabbit and human antibodies against human factor VIII to Sepharose CL-2B. The resulting insoluble antibodies completely removed factor VIII and fibrinogen from normal human citrate plasma. Other coagulation factors were satisfactorily recovered in the eluted plasma. Following addition of fibrinogen, the factor VIII-deficient plasma was used for calibration of the one-stage factor VIII assay and compared with the hemophilic plasma. Parallel straight lines were obtained in the log-log plot against VIII:C, indicating that the artificial reagent can be used as a substitute for hemophilic plasma in determination of factor VIII procoagulant activity. The immunoadsorbent can be regenerated and repeatedly used for affinity chromatography binding of factor VIII.

Interaction study between phenprocoumon and flurbiprofen

The effects of flurbiprofen on various coagulation and haemostasis parameters were studied in 19 patients adequately anticoagulated on a fixed dose of phenprocoumon. Laboratory controls were performed 5 times at weekly intervals. Flurbiprofen 50 mg 3-times daily was given for 2 weeks after an initial control period of 2 weeks. The last control followed 1 week after cessation of flurbiprofen. All changes observed during the administration of flurbiprofen were quantitatively moderate; some reached statistical significance. The prothrombin time (%) Factor II and Factor X values fell; Factor II values remained unchanged. Factor IX values fell only in the first week on flurbiprofen and reached control values thereafter. Phenprocoumon concentration remained constant throughout. There was no change in spontaneous platelet aggregation. Although there was a slight, statistically significant increase in the Ivy bleeding time, it remained within the normal range. These findings suggest that the interference of flurbiprofen with oral anticoagulation is minimal and probably of no clinical relevance.