Standardization of Factor VIII. I. Calibration of British Standards for Factor VIII clotting activity

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