Extended Data Fig. 1: Activation of platelets in samples from patients with VITT and patients with HIT in different SRAs.

Platelet activation is shown as the percentage release of ¹⁴C-serotonin. a, c, Samples from patients with VITT (n = 5; red lines) (a) and patients with HIT (n = 10; blue lines) (c) in the standard SRA²⁷. b, d, The same VITT (b) and HIT (d) samples in the PF4-enhanced SRA^28,31 with added PF4. The results showed that in the standard SRA, the HIT samples activated platelets with (n = 4) or without (n = 6) heparin or with added PF4, whereas the VITT samples only activated platelets with the addition of PF4. Experiments were repeated with technical duplicates independently twice with similar results. All platelet activation was inhibited by the addition of the IV.3 monoclonal antibody that binds the FcγRIIa receptors on platelets.