Exploratory cohort of patients with strong reactivity in the in-house anti-PF4/heparin IgG EIA but negative heparin-dependent functional assay (n = 188). 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). Of these 188 patients, 40 (21.3%) tested positive by the new rapid anti-PF4 assay (upper left and right quadrant). Of those with sufficient available serum (n = 33, dark blue), 13 (39.4%) tested positive by PF4-dependent platelet activation assay (dark blue with red outline; 4 in the upper left quadrant and 9 in the upper right quadrant). ASA, acetylsalicyl acid; DVT, deep vein thrombosis; LMWH, low molecular weight heparin; PE, pulmonary embolism; VKA, vitamin K antagonists.