Plasma VWF:Ag levels on follow-up testing compared with VWF:Ag on entry samples. VWF testing was done at the time of original enrollment in the ZPMCB-VWD. After several years, repeat testing was performed. Samples included index cases, family members, and some normal controls to evaluate the changes over time. The correlation coefficient was r² = 0.68, with fairly comparable results in those whose VWF levels were <50 IU/dL.