title: Calibration of lyophilized standards for ristocetin cofactor activity of von Willebrand Factor (vWF) requires vWF-deficient plasma as diluent for dose-response curves.
abstract: Calibration of local standards for ristocetin cofactor activity of von Willebrand's factor (vWF:Rcof) against reference preparations is required to achieve a better standardization of this measurement. We have observed that in calibration assays (multiple line parallel bioassay) for vWF:Rcof, measured aggregometrically, the type of diluent used to obtain the dose-response curves exerted a critical influence on the final potency estimation when a fresh-frozen plasma against a lyophilized standard or two lyophilized preparations were to be compared. This pattern was observed with various commercial standards and also with the First International Reference Preparation for Factor VIII-Related Activities in Plasma. The choice of diluent was not relevant if two fresh-frozen plasma preparations were to be calibrated. Systematic experiments using as diluents Tris-saline, Tris-buffered-albumin solution or vWF-deficient plasma (vW-plasma) showed that the ratio lyophilized/fresh-frozen preparations was invariably lower with albumin solution than with vW-plasma. Tris-saline partially corrected this discrepancy, but yielded less precise estimates. Preliminary results with cryosupernatant-plasma seems to indicate that this material is a valid substitute for vW-plasma, but confirmatory experiments are required. For the moment, plasma obtained from patients with severe type III vW-disease should be considered the "ideal" diluent for calibration assays involving lyophilized preparations.
mesh_terms: D005612:Freeze Drying; D006801:Humans; D012015:Reference Standards; D014842:von Willebrand Diseases / Q000097:blood; D014841:von Willebrand Factor / Q000032:analysis / Q000592:standards*
chemical_list: D014841:von Willebrand Factor