Ibrutinib, but not acalabrutinib, causes thrombus instability. Whole blood from healthy donors was incubated with vehicle, 1 µM ibrutinib, and 1 µM acalabrutinib for 20 minutes and flowed through collagen-coated microfluidic flow chambers at a shear rate of 1000 s⁻¹ for 6 minutes. (A) Representative images of thrombi formed under each of the conditions tested at 2-minute intervals. (B) A summary of effects of 1 µM ibrutinib and 1 µM acalabrutinib on thrombus formation on collagen and other platelet signaling and functional measurements relative to vehicle-treated control. Bars represent the mean ± SD; P values were calculated using multiple tests with Holm-Sidak correction for multiple comparisons.