Thrombus formation on collagen type I surface under high shear. The closure time on the PFA-100 was measured in triplicate with 8 “aa” (▪) and 6 “bb” (▦) donors by means of both the COL/EPI (collagen type I and epinephrine) and the COL/ADP (collagen type I and ADP) cartridges. Data are expressed as the mean ± SEM and tested by unpaired t test.