Platelet binding of IgG in selected SRA-positive (filled circles) and SRA-negative (open circles) HIT samples in the presence and absence of PF4. Ordinate depicts fold increase in IgG binding relative to value obtained with normal serum. Each circle depicts the average of triplicate measurements. Horizontal bars depict the mean of each group. Asterisks indicate significance of the difference between means (**P < .01). Insert summarizes the results of PF4 ELISA testing done with the same sample set. SRA-negative and SRA-positive samples did not differ from each other in the PF4 ELISA.