VITT–IgG-mediated procoagulant platelets and platelet activation in the presence of anticoagulants. (A) Procoagulant platelets (CD62P/PS-positive cells) were analyzed with flow cytometry after incubation of healthy platelets with VITT sera or sera from HCs in different settings and staining with annexin V–FITC and CD62p–APC Ab. Where indicated, platelets were coincubated with different concentrations of anticoagulants: heparin (unfractionated heparin, 0.1, 0.5, 1, 5, and 10 U/mL), danaparoid (0.8 and 8 U/mL), fondaparinux (1 and 8 µg/mL), and argatroban (0.8 and 8 µg/mL). The percent of CD62p/PS-double–positive platelets is shown as violin plots. Each dot represents sera from an individual VITT patient, and the number of sera tested is reported in each graphic. Violin plots showing the distribution of the values were generated using Graphpad Prism 8. ns, not significant. *P < .05, **P < .01, ***P < .001, and ***P < .0001. (B) Results of the platelet activation assay (HIPA). Each dot represents the median value of 2 different healthy platelet donors. VITT patients showed strong platelet activation with buffer alone in comparison with HCs, which was inhibited with varying concentrations of heparin. Where indicated, platelets were treated with different concentrations of anticoagulants: heparin (unfractionated heparin, 0.2, 1, 10, and 100 U/mL), danaparoid (0.8 U/mL), fondaparinux (8 µg/mL), and argatroban (8 µg/mL). Data are presented as time to aggregate. Violin plots showing the distribution of the values were generated using Graphpad Prism 8. *P < .05 and **P < .01.