The TFPI K2 domain is detected by immunoassay on APC-treated but not factor Xa-treated cells. Cells were treated with various concentrations of APC and factor Xa for 3 hours followed by cell surface immunoassay using the indicated antibodies. Thrombin (5nM) was present during the 3-hour incubation in panels B and C where indicated. Means ± SEM with n = 6 are shown, *P < .05; **P < .005, compared with no APC/Xa.