Impact of sera from COVID-19 patients on platelet apoptosis. (A-C) Changes in apoptosis pathways induced by sera from COVID-19 patients in the ICU, non–COVID-19 patients in the ICU (control group), and healthy donors were investigated by assessing the depolarization of the ΔΨm (A), cytosolic calcium concentration (B) (n = 19 because of the lack of biomaterial for 2 patients), and PS externalization (C). (D) Quantification of cleaved caspase 9 level in platelets from healthy donors after incubation with patient sera, normalized to sera from healthy donors. (E) Representative western blot of GAPDH and cleaved caspase 9 proteins. Protein bands were detected with the infrared imaging system (Odyssey, LI.COR). (F) Diagram indicating the number of COVID-19 patients in the ICU positive for each apoptotic parameter: ΔΨm depolarization, cytosolic calcium concentration, and PS externalization. The 2 sera tested only for ΔΨm depolarization and PS externalization are in parentheses. Data are presented as mean ± SEM of the measured FI compared with control. Not significant, *P < .05, **P < .01, ***P < .001, ****P < .0001. The number of sera tested is reported in each graphic. Dashed lines represent the cutoffs determined testing sera from healthy donors as mean of FI + 2× SEM.