VWF reduces the measurement of free PS. (A) Free PS and total PS enzyme-linked immunosorbent assay (ELISA) measurements of HNP (from Siemens), with additions of purified VWF, under shear (∼2500 rpm for 30 seconds). Free and total PS are presented as percent of free and total PS in reference plasma, respectively. (B) Free PS ELISA measurements of healthy control plasmas from single donors with or without shear. P values are according to Wilcoxon matched-pairs signed rank test; ∗∗P < .01. (C) Free PS ELISA results of purified proteins (200 nM PS and 10 μg/mL VWF in HEPES-buffered saline, with albumin), HNP1 (Corgenix), and HNP2 (Siemens), with or without shear or 1 μM hirudin, 5 mM Gly-Pro-Arg-Pro peptide, and 5 mM CaCl₂ supplementation; nondetectible. P values for panel A are according to 2-way ANOVA with Sidak multiple comparisons test. P values for panels B-C are according to Kruskal-Wallis with Dunn multiple comparison test; ∗P < .05; ∗∗P < .01. OD, optical density; rpm, revolutions per minute.