Standardization of Factor VIII I. CALIBRATION OF BRITISH STANDARDS FOR FACTOR VIII CLOTTING ACTIVITY

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Abstract
Calibration of successive British Standards for Factor VIII clotting activity against the International Standard (concentrate) has revealed substantial discrepancies among lab oratories and between assay methods. These discrepancies were less in assays of concentrate standards than in calibration of plasma standards. Standardization of reagents in the 2-stage assays substantially improved agreement among laboratories using this method. Standardization of the phospholipid reagent and hemophilic substrate separately had little effect on variation among laboratories performing 1-stage assays. Standardization of reagents did not alter the basic discrepancy (.apprx. 20%) between the 1-stage and 2-stage assay methods. Omission of the aluminium hydroxide adsorption step from the 2-stage method reduced the discrepancy between the 2 methods to less than 10%. Discrepancies did not occur when 1 plasma standard was assayed against another. Improvements in the stability of VIII:C [factor VIII: procoagulant] in freeze-dried plasma make it feasible to establish a long-term plasma reference standard.