von Willebrand factor standards for plasma and concentrate testing

The wide range of plasma von Willebrand factor (vWF) levels in the normal population makes vWF an ideal candidate for calibration in World Health Organization International Standards (WHO IS). Calibration of the WHO IS Plasma for vWF:antigen (vWF:Ag), vWF:ristocetin cofactor activity (vWF:RCo), and vWF:collagen-binding activity (vWF:CB) addresses the need to reconcile the original definition of the International Unit (IU) in fresh normal plasma with continuity between replacement freeze-dried preparations, whereas calibration of the WHO IS Concentrate aims to maintain equivalence between the IU used for the estimation of vWF in plasma (diagnosis) and the IU used for the potency labeling of therapeutic concentrates (replacement therapy). The latter objective was achieved through the calibration of the WHO 1st IS vWF Concentrate, for vWF:Ag and vWF:RCo, by assay directly relative to the WHO IS Plasma. Calibration of the WHO IS Concentrate for vWF:CB was not possible due to large interlaboratory and intermethod discrepancies, and resolution of this problem may require the use of recommended methodology. There is evidence that the use of the WHO IS Concentrate leads to considerable reduction in interlaboratory variability for vWF estimates in therapeutic concentrates and hence the "assay like versus like" principle should be applied.