Platelet reactivity following stimulation in patients with severe COVID-19 compared with healthy donors. Platelet-rich plasma from 15 healthy donors and 16 patients with severe COVID-19 was stimulated or not with TRAP (50 µM), U46619 (5 µM), and CRP (0.9 µg/mL) for 10 minutes in nonstirring conditions at 37°C, and activation of αIIbβ3 (GpIIb-IIIa) was assessed by flow cytometry using PAC-1 antibody (A). The number of copies of surface GpIIb-IIIa in resting platelets was also quantified (B, n = 8 healthy donors and n = 9 patients). The surface expression of P-selectin (CD62P), a marker of α-granules secretion (C), and CD63, a marker of dense granules secretion (D), was quantified in resting and stimulated platelets. The platelet aggregation response (% of maximal platelet aggregation) from healthy donors and patients with severe COVID-19 was also assessed by light transmission aggregometry in response to TRAP (50 µM), U46619 (1 µM), and collagen (0.75 µg/mL) for 10 minutes in stirring conditions (E). Results are mean ± SEM; each circle represents an individual. *P < .05; **P < .01; ***P < .001; ****P < .0001 according to the nonparametric Mann-Whitney test.