Background Severe coronavirus disease 2019 (COVID‐19) is characterized by an increased risk of thromboembolic events, with evidence of microthrombosis in the lungs of deceased patients. Objectives To investigate the mechanism of microthrombosis in COVID‐19 progression. Patients/Methods We assessed von Willebrand factor (VWF) antigen (VWF:Ag), VWF ristocetin‐cofactor (VWF:RCo), VWF multimers, VWF propeptide (VWFpp), and ADAMTS13 activity in a cross‐sectional study of 50 patients stratified according to their admission to three different intensity of care units: low (requiring high‐flow nasal cannula oxygenation, n = 14), intermediate (requiring continuous positive airway pressure devices, n = 17), and high (requiring mechanical ventilation, n = 19). Results Median VWF:Ag, VWF:RCo, and VWFpp levels were markedly elevated in COVID‐19 patients and increased with intensity of care, with VWF:Ag being 268, 386, and 476 IU/dL; VWF:RCo 216, 334, and 388 IU/dL; and VWFpp 156, 172, and 192 IU/dL in patients at low, intermediate, and high intensity of care, respectively. Conversely, the high‐to‐low molecular‐weight VWF multimers ratios progressively decreased with increasing intensity of care, as well as median ADAMTS13 activity levels, which ranged from 82 IU/dL for patients at low intensity of care to 62 and 55 IU/dL for those at intermediate and high intensity of care. Conclusions We found a significant alteration of the VWF‐ADAMTS13 axis in COVID‐19 patients, with an elevated VWF:Ag to ADAMTS13 activity ratio that was strongly associated with disease severity. Such an imbalance enhances the hypercoagulable state of COVID‐19 patients and their risk of microthrombosis.

Willebrand factor dramatically enhances the multimer sensitivity to shear stress. J Thromb Haemost 2013; 11: 1688-98.Summary. Background: Shear stress triggers conformational stretching of von Willebrand factor (VWF), which is responsible for its self-association and binding to the platelet receptor glycoprotein (GP)Iba. This phenomenon supports primary hemostasis under flow. Type 2B VWF natural mutants are considered to have increased affinity for platelet GPIba. Objectives: To assess the mechanism responsible for the enhanced interaction of the p.R1306W VWF mutant with the platelet receptor. Methods: The interaction of GPIba with wild-type (WT) and p.R1306W VWF multimers and A1-A2-A3 constructs was investigated with surface plasmon resonance spectroscopy. Analysis of the static VWF conformation in solution was performed with dynamic light scattering spectroscopy. The shear stress-induced self-association of VWF multimers was investigated with atomic force microscopy (AFM) over a 0-60 dyn cm À2 range. Results: WT VWF did not interact with GPIba under static conditions, whereas the mutant at~2 lg mL À1 already bound to the receptor. By contrast, the WT and p.R1306W-A1-A2-A3 constructs showed comparable affinities for GPIba (K d~2 0 nm). The hydrodynamic diameter of resting R1306W VWF multimers was significantly greater than that of the wild type (210 AE 60 nm vs. 87 AE 22 nm). At shear forces of < 14 dyn cm À2 , the p.R1306W multimers rapidly changed conformation, entering a regime of selfaggregation, which, in contrast, was induced for WT VWF by shear forces of > 30 dyn cm À2. Mechanical stretching AFM experiments showed that p.R1306W multimers needed less energy per length unit (~10 pN) to be stretched than the WT protein. Conclusions: The increased affinity of p.R1306W VWF for GPIba arises mostly from higher sensitivity to shear stress, which facilitates exposure of GPIba binding sites.

In individuals with borderline VWF, the two strongest predictors of VWD diagnosis are low VWF:RCo levels (particularly in those with blood group non-O) and female sex. This predictive model has a promising discriminative ability to identify patients with borderline VWF levels who are likely to have VWD.

Introduction von Willebrand disease (VWD) diagnosis starts with first level tests: factor VIII coagulant activity, VWF antigen (VWF:Ag) and platelet‐dependent VWF activity (VWF:RCo, VWF:Ab, VWF:GPIbR or VWF:GPIbM). The VWF collagen binding (VWF:CB) assay measures the binding capacity of von Willebrand factor (VWF) to collagen. Aim To assess, in previously diagnosed VWD patients, the performance of a fully automated chemiluminescent test panel including VWF:Ag, VWF:GPIbR and VWF:CB assays. Methods The patients, historically evaluated using in‐house VWF:Ag and VWF:CB assays and an automated latex enhanced immunoassay VWF:GPIbR method, were re‐evaluated using the VWF test panel HemosIL AcuStar. Results The VWF:GPIbR/VWF:Ag and VWF:CB/VWF:Ag obtained by means of AcuStar showed an overall good concordance with the corresponding data obtained at the time of the historical diagnosis. When discrepancies occurred, these were generally due to the lower VWF:CB/VWF:Ag obtained with AcuStar as compared with that obtained with the historical methods and this affected particularly the diagnosis of VWD type 2M. Together, the AcuStar VWF:GPIbR/VWF:Ag and VWF:CB/VWF:Ag were able to distinguish type 1 from types 2A, 2B and 2M, whereas no distinction was possible between type 2A and 2B. Conclusion The AcuStar panel offers a good performance in the differential diagnosis between VWD type 1 and 2A/2B patients. A high rate of coincidence with historical diagnosis was obtained for VWD types 3, 2A/2B and 1. Even though in some cases more tests (eg, RIPA/multimeric analysis) are needed to complete an accurate VWD classification, the AcuStar panel is considered a sensitive, rapid and reliable tool to diagnose VWD patients.