Platelet binding of IgG in 57 identity-blinded HIT antibody samples in the presence and absence of PF4 and HD heparin. (Left) In the absence of added PF4, IgG from SRA-negative samples (open circles) failed to bind, and SRA-positive samples (filled circles) bound very weakly. (Center) When PF4 was present, binding of IgG in SRA-positive samples, but not SRA-negative samples, was markedly enhanced. (Right) PF4-dependent IgG binding was completely inhibited by HD heparin (100 U/mL). Ordinate depicts fold increase in IgG binding relative to value obtained with normal serum. Each circle depicts the average of duplicate determinations. Horizontal bars depict the mean of each group. Asterisks indicate significance of the difference between means (***P < .001; *P < .05).