Sigmoidal dependence of VWF cleavage on platelet count. Reactions containing VWF (30 μg/mL), without (−) or with (+) ADAMTS13 (2.5 U/mL), and various concentrations of platelets were exposed to fluid shear stress (50 dyne/cm², 3 minutes) and analyzed by SDS-PAGE and Western blotting. (A) Examples of platelet dose response results with formalin-fixed platelets (top) and fresh washed platelets (bottom). (B) Data with fixed platelets were combined by normalizing the density of the 350-kDa product band to the density of a cleavage product standard (Std). Error bars indicate SE (n = 9).