STANDARDS FOR FACTOR VIII

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Abstract
Because of the need to relate haemophilic samples to the normal population, the unit of Factor VIII clotting activity (VIII:C) was first defined as the amount in 1 ml. of normal plasma. However, because of wide variation in the normal population, and poor stability of VIII:C in plasma, pooled normal plasma is unsuitable as a reference standard for Factor VIII. The 1st International Standard for Factor VIII was established in 1970 to provide a stable reference material. It consisted of a freeze‐dried intermediate‐purity concentrate, and its potency was established by comparison with fresh normal plasma in a large number of laboratories. This standard was replaced in 1976 by the 2nd International Standard, also a stable intermediate purity concentrate. These long‐term reference standards are intended for calibration of working standards, and for manufacturers of concentrates it is preferable to use concentrates as working standards, because of the desirability of assaying «like against like». For assay of patients' plasmas, a plasma standard is necessary, and in the U.K. successive batches of British plasma standards have been calibrated against the International Standard (concentrate) by several laboratories. However, calibration of these plasmas against the concentrate standard has been shown to give substantial discrepancies between laboratories, and an overall discrepancy of 20% between one‐stage and two stage assays. Although standardisation of reagents and technique has improved agreement between laboratories, discrepancies still remain, and these could be markedly diminished by establishment of a plasma reference standard, to exist alongside the concentrate standard. Improvements in methods of handling have led to better stability of Factor VIII in freeze‐dried plasma and it is now planned to establish a stable reference plasma for VIII:C. Such a plasma could also act as a reference for the other Factor VIII related activities, i.e. VIII related antigen, VIII clotting antigen, and VIII ristocetin co‐factor.

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