Platelet activation pattern induced by TTS sera. Platelet activation induced by sera from 18 patients with anti-PF4/heparin ELISA–negative TTS. Control sera (n = 6) were obtained from healthy blood donors. Platelets of healthy donors were washed and incubated with the patients’ sera. Each dot represents the mean of at least 3 experiments. (A) Procoagulant platelet formation and (B) CD62P expression was assessed in flow cytometry. Activation was blocked by the monoclonal antibody IV.3, which inhibits platelet activation via FcγRIIa and by RGDS (blocks signaling of GPIIb/IIIa). Cutoff was defined as mean + 3 standard deviations of negative control (shown in beige). Negative control: phosphate-buffered saline; positive control: 20 μM thrombin receptor activating peptide + 100 ng/mL convulxin. Statistics were performed using an ordinary 1-way analysis of variance followed by an uncorrected Fisher least significant difference test, ∗P < .05; ∗∗P < .01; ∗∗∗P < .001. MFI, mean fluorescence intensity; PBS, phosphate-buffered saline; TRAP/Conv, thrombin receptor activating peptide + convulxin.