VWF and endothelial dysfunction: However, unlike most low-flow states such as deep venous thrombosis where erythrocyte-rich thrombi prevail, in AF platelets contribute significantly to thrombus formation and the endothelial dysfunction which exists, suggests that an in vitro model like the GTT, where platelets and von Willebrand factor are key players and where endogenous fibrinolysis is assessed, may provide a useful assessment of this milieu.