Elevated hematocrit increases the rate of platelet accumulation on collagen at an arterial shear rate. Reconstituted whole blood was perfused over type I collagen at 750 seconds⁻¹. (A) Representative images of thrombus surface coverage and platelet fluorescence 1.5 minutes after initiation of flow. Scale bar indicates 50 µm. (B) Average thrombus area (normalized to collagen spot area as fractional surface coverage) and individual thrombus lag times and rates for all experiments. (C) Average platelet accumulation (integrated fluorescence intensity units [IFU] of DiOC₆-labeled platelets) and individual platelet lag times and rates for all experiments. N = 7 independent donors per condition. Values show mean ± SD for 41% (open circles) and 54% (closed circles) hematocrit; lines indicate paired data for individual blood donors.