The evaluation of factor VIII antigen by means of a simple slide test

Factor VIII von Willebrand may be detected by a Ristocetin-dependent platelet agglutination reaction. The authors have measured plasma levels of Factor VIII-related Ristocetin cofactor (VIIIR:RCoF) using formalin-fixed platelets tested by an aggregometric method or by a new, commercially available glass slide test. Plasmas were obtained from patients with von Willebrand's disease and Cushing's syndrome, and from normal controls. Von Willebrand factor (vWf) levels varied between 0-500% of the normal. A good correlation (r = 0.95) was found comparing the results obtained by the two methods over the entire range. If all patients were considered, a satisfactory correlation was obtained between VIIIR:RCoF and VIII-related antigen (VIIIR:Ag), both by the aggregometric method (r = 0.79) and by the glass slide method (r = 0.88). On the contrary, if values below 25% of the normal are taken into account, only a mild correlation was found between VIIIR: RCoF (aggregometric method) and VIIIR:Ag and no correlation between VIIIR:RCoF (glass slide method) and VIIIR:Ag. No correlation, regardless of the method used, was found between VIIIR:RCoF and VIIIR:Ag for values above 120%.