Platelet activation induced by 57 identity-blinded HIT antibody samples in the presence and absence of PF4 and HD heparin. (Left) In the absence of added PF4, minimal, but significant, p-selectin expression was induced by SRA-positive samples (filled circles) only. (Center) When PF4 was present, p-selectin expression induced by SRA-positive samples was markedly enhanced. (Right) PF4-dependent p-selectin expression was significantly inhibited by HD heparin (100 U/mL). Ordinate depicts fold increase in p-selectin expression relative to value obtained with normal serum. Values shown are the means of duplicate determinations. Horizontal bars depict the mean of each group. Asterisks indicate significance of the difference between means (***P < .001; **P < .01; *P < .05).