Excess CS and heparin (relative to PF4) inhibit PF4-dependent binding of KKO and HIT IgG to platelets and resultant platelet activation. Platelets were treated with PF4 alone (black bar), PF4 and excess heparin (white bar), or PF4 and excess CS (gray bar), and with KKO (left) or HIT sera (right). Values shown are the average of triplicate determinations + 1.0 standard deviation. The ordinate depicts median fluorescence intensity of IgG binding to platelets (A) and platelet p-selectin expression (B). IC, isotype control; NC, pooled normal serum. Asterisks indicate significance of differences between means of samples treated with excess GAG vs no GAG (***P < .001; **P < .01; P < .05).