Free PS deficiency in patients with COVID-19 can be explained by changes in VWF but not C4BP. Citrated plasma samples were collected from healthy controls (n = 38) and patients with COVID-19 (n = 30). Due to sample limitations, some measurements could not be performed with all participants. (A-D) Total PS (A), free PS (B), C4BP-β (C), and VWF (D) were measured by ELISA. (E) Free PS plotted against VWF:Ag. (F-J) Plasma thrombin generation was measured using calibrated automated thrombography. Several patients were receiving heparin prophylactic dose at the time of blood collection. Shown are the ETPs in assays initiated with 4 μM phospholipids and 1 pM tissue factor (TF) (F) or with 20 nM thrombomodulin (TM) supplementation to evaluate the contribution of APC/PS activity (G), and the ETP ratio (value with TM/value without TM) (H). (I-J) Free PS plotted against ETP in the absence (I) or presence (J) of thrombomodulin. Every data point is the average of 3 replicates (mean ± SD). For panels A-H, P values are according to Mann-Whitney test; ∗P < .05; ∗∗P < .01. For panels I-J, P values and r correlation coefficients are according to Spearman correlation analysis; ∗∗∗P < .001; ∗∗∗∗P < .0001. Blue squares represent controls, and red circles represent patients with COVID-19. FPS, free protein S; TPS, total protein S.