ABSTRACT
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.
KEYWORDS
von Willebrand factor (vWF) - International Standard - ristocetin cofactor - collagen
binding
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Dr.
Anthony R Hubbard
Division of Haematology, National Institute for Biological Standards & Control
Blanche Lane, South Mimms, Potters Bar, Hertfordshire, EN6 3QG United Kingdom
eMail: thubbard@nibsc.ac.uk
