Rapid assay results of patient sera with HIT, VITT, and negative controls. Results of the rapid anti-PF4/heparin assay are given on the x-axis (cutoff: dashed vertical line). Results of the new rapid anti-PF4 assay are given on the y-axis (cutoff: dashed horizontal line). Sera of patients with heparin-induced thrombocytopenia are shown in red (n = 131), VITT sera in blue (n = 103), and negative controls in gray symbols (n = 155). None of the 16 VITT sera reacting positive with the rapid anti-PF4/heparin assay (blue dots upper right quadrant) tested positive with 0.2 aFXaU heparin in the heparin-dependent platelet activation assay (12 undiluted; 4 at an informative serum dilution [giving negative buffer reactivity]).