Use of Venom Coagglutinin and Lyophilized Platelets in Testing for Platelet-Aggregating von Willebrand Factor

Previous procedures for measuring von Willebrand factor (vWF) in human plasma have usually utilized ristocetin and platelet-rich plasma or washed platelets. In this study, we describe a new approach for determination of vWF utilizing two new reagents. One is venom coagglutinin, a recently recognized component of Bothrops and other venoms, which aggregates platelets in the presence of vWF. The other is lyophilized fixed human platelets. The rate of platelet aggregation was found to be a function of both plasma vWF and venom coagglutinin concentration. By keeping the venom factor constant, simple and reliable test procedures for vWF were devised. Both a macroscopic test procedure and aggregometry were used for measuring platelet aggregation, with similar findings. Results obtained with the venom coagglutinin reagent were compared with those obtained with ristocetin. With the venom factor, normal plasmas from 20 subjects had a range of 70%–133% vWF and a mean of 96% of a reference plasma, while with ristocetin, the values were 63%–131%, mean 101%. Plasmas from seven von Willebrand disease patients gave comparable reduced values for vWF with venom factor and with ristocetin. The venom coagglutinin and dried platelet reagents appear to provide the basis for simple and reliable procedures for determination of plasma vWF.