RBCs increased platelet thrombus formation on collagen under flow conditions. Human blood reconstituted to varying hematocrits (HCTs) and platelet counts was perfused over a collagen-coated surface using a flow perfusion chamber at a shear stress of 2 Pa. The proportion of surface area covered by platelet thrombus was calculated and is expressed as mean thrombus surface percentage ± standard deviation. (A-C) Representative images of platelet thrombus formed on immobilized collagen after 5-minute perfusion (scale bar, 50 μm) and collective data from multiple independent experiments with different platelet counts and HCTs (n = 49-50 per experimental condition; Student t test). (D) Representative images of thrombus surface coverage (scale bar, 50 μm) after 10-minute perfusion of blood reconstituted to a platelet concentration of 10 × 10³/μL and varying HCT (n = 50-59 per experimental condition; 1-way analysis of variance [ANOVA] and Tukey honestly standard deviation).