Impaired P-selectin expression on fibrinogen-deficient human platelets. Cell surface P-selectin expression was detected on thrombin- or TRAP-activated healthy control, heterozygous, and homozygous platelets using flow cytometry. A significant decrease in P-selectin expression on the surface of heterozygous and homozygous platelets was observed compared with healthy controls after thrombin treatment (left panel, MFI ± SEM, **P < .001) or TRAP treatment (right panel, MFI ± SEM, **P < .001). The experiments were repeated at least 3 times.