Thrombus formation on collagen correlates with platelet count during treatment with acalabrutinib but not with ibrutinib. (A) Blood from healthy donors (n = 6), Btk-inhibitor naïve CLL patients (n = 5), patients receiving ibrutinib (n = 6) or acalabrutinib (n = 8) was perfused through collagen-coated microfluidic flow chambers at room temperature at a shear rate of 1000 s−1 before being fixed with 10% formyl saline. Fixed samples were stained with DioC6 and z-stack images acquired to enable estimation of thrombus volume. (A) Representative images of thrombi in blood from a healthy donor and patients in the presence or absence of Haemate P (intermediate factor VIII) added ex vivo. Thrombus volume plotted against platelet count for (B) Btk inhibitor naïve CLL patients; patients treated with (C) acalabrutinib or (D) ibrutinib with or without added Haemate P. Correlation between platelet count and thrombus volume is significant in the presence or absence of Haemate P for acalabrutinib patients (significantly nonzero, P ≤ .05) but nonsignificant for ibrutinib patients (P > .05).