Increased platelet activation and TF expression by monocytes associate with severity and mortality in COVID-19. (A-E) Severe COVID-19 patients admitted to the ICU were stratified between those requiring invasive mechanical ventilation or noninvasive O₂ supplementation. The percentages of P-selectin (CD62P) (A) and CD63 (B) expression on platelets, the concentration of TXB₂ in plasma (C), the percentage of monocytes forming aggregates with platelets (D), and TF expression on monocytes (E) are shown. (F-K) Severe COVID-19 patients were stratified according to the 28-day mortality outcome as survivors or nonsurvivors. The percentages of P-selectin (CD62P) (F) and CD63 (G) expression on platelets, the concentration of TXB₂ in plasma (H), the quantification of PDGF-BB in tracheal aspirates (I), and the percentage of monocytes forming aggregates with platelets (J) and expressing TF (K) are shown. The horizontal lines in the box plots represent the median, the box edges represent the interquartile ranges, and the whiskers indicate the minimal and maximal value in each group. The gray box indicates the interquartile ranges and the dotted line represents the median of the SARS-CoV2⁻ control group. *P < .05 between selected groups.