Platelet activation markers in patients with severe COVID-19 compared with controls. Soluble markers of platelet activation including soluble P-selectin (A), soluble GPVI (B, left panel), and soluble CD40L (C) were quantified in the plasma from patients with severe COVID-19 and healthy donors (A: n = 12 healthy donors and n = 6 patients; B: n = 20 healthy donors and n = 22 patients; C: n = 11 healthy donors and n = 15 patients). The number of copies of platelet surface GPVI was also quantified (B, right panel, n = 16 healthy donors and n = 15 patients). Eicosanoids (D) known to be produced by activated platelets, including TXB2, 12-HETE, 14-HDoHE, 15-HETE, and 5-HETE, were quantified in plasma from 7 patients with severe COVID-19 and 6 healthy donors using a mass spectrometry-based targeted lipidomic approach. The presence of heterotypic monocyte-platelet (E) and neutrophil-platelet (F) aggregates were quantified by flow cytometry in 13 patients with severe COVID-19 and 11 healthy donors. The concentrations of IL-1beta, IL-6, and IL-8 were also quantified using appropriate enzyme-linked immunosorbent assay kits (G) in the plasma of 6 to 10 patients with severe COVID-19 and 4 to 8 healthy donors. Results are mean ± SEM; each circle represents an individual. *P < .05; **P < .01; ***P < .001; ****P < .0001 according to the nonparametric Mann-Whitney test.