Measurement of platelet von Willebrand factor is dependent on method of platelet isolation

The authors have compared platelet von Willebrand factor antigen and ristocetin cofactor activity measurements in a normal population, using two different previously published platelet isolation techniques. Preparation of a platelet lysate by platelet washes using Tyrode's albumin solution and inhibitors yielded a threefold higher vWF antigen and a twofold higher ristocetin cofactor activity measurement as compared to platelets isolated by tris buffered saline washes containing 0.1% (weight/volume) Na2EDTA. Contamination by plasma von Willebrand factor to account for the higher values found by the first method was excluded by monitoring the presence of added purified I125 von Willebrand factor to platelet rich plasma. vWF multimer analysis showed the same distribution of multimers in both preparations but suggested a relative decrease in lower molecular weight multimers with the second method. Platelet isolation can result in significant loss of platelet von Willebrand factor, with a preferential loss of lower molecular weight multimers, likely resulting from unintentional platelet release.