Formation of dense fibrin clots in plasma of patients with severe COVID-19. (A) Western blot analysis of factor XII in plasma of patients with ARDS-influenza (Influenza) or COVID-19 (SARS-CoV-2), as well as donors. Data from 4 of 21 patients with COVID-19, 4 of 25 patients with ARDS-influenza, and 3 of 21 donors are shown. Albumin was used as the loading control. (B) FXII levels in plasma of patients with ARDS-influenza (n = 25) or COVID-19 (n = 21) and donors (n = 16) as assessed by enzyme-linked immunosorbent assay. (C) Lag phase in fibrin formation triggered by kaolin. Influenza, n = 19; SARS-CoV-2, n = 20; donor, n = 20. (D) FVIII activity (FVIII:C) in plasma of patients and donors. Influenza, n = 19; SARS-CoV-2, n = 20; donor, n = 20. (E-F) Time to reach the turbidity peak (E) and maximum (Max) turbidity (F) values for influenza (n = 19), SARS-CoV-2 (n = 20), and donor (n = 20) plasma. Clot formation was induced by the addition of kaolin to plasma. (G) Representative laser scanning confocal microscopy images of fibrin fibers in clots formed from influenza (n = 19), SARS-CoV-2 (n = 20), and donor (n = 20) plasma. Fibrin fibers were stained with an anti-fibrinogen/fibrin antibody followed by an Alexa Fluor 488-conjugated secondary antibody. (H) Representative scanning electron microscopy images of fibrin network in clots generated from influenza plasma (n = 5) as well as low- and high-fibrinogen SARS-CoV-2 (n = 5 per group) plasma. (I) Fibrin fiber density in donor (n = 20), ARDS-influenza (n = 19), and COVID-19 (n = 20) clots. From each patient, 3 separate clots were prepared, 5 images were taken in different areas of the clots, and fibril density was determined in all images. (J) Correlation between maximum turbidity values and fibrinogen levels in plasma of patients with COVID-19 (n = 15; those patients with available fibrinogen levels were included into the analysis). Correlation was performed according to Spearman’s rank correlation coefficient.