VWF and paroxysmal nocturnal hemoglobinuria: We further show in this study that Hb concentrations ranging from 5 to 10 μM, commonly observed level in plasma of PNH patients [1] significantly increases thrombus formation in vitro on immobilized VWF under arterial shear stress (Fig 1A, 1B4 and 1B5) but not under lower shear stress such as 5 dyne/cm2 that simulate venous blood flow (Fig 1A, 1B1–1B3).