Enzyme capture assay for FXIa. (A) Effect of benzamidine on the activity of purified FXIa in the amidolytic assay and in the ECA. Microtiter plates coated with MoAb XI-5 were incubated with various concentrations of FXIa in the presence or absence of benzamidine (100 mmol/L, final concentration). The plates were washed extensively and incubated for 72 hours with S-2366. Results obtained were expressed as the change in absorbance at 405 nm and compared with those obtained with an amidolytic assay in which FXIa was incubated in the presence or absence of benzamidine (100 mmol/L, final concentration) with S-2366. The means of four experiments ± SD are given. FXIa in the ECA incubated in the absence (○) or in the presence (•) of benzamidine. FXIa directly tested in the amidolytic assay in the absence (□) or in the presence (▪) of benzamidine. (B) Recovery in the ECA of purified FXIa added to plasma. Dilutions of purified FXIa were added to FXI-deficient plasma or EDTA plasma containing 100 mmol/L benzamidine. Samples were tested in the ECA as described in Materials and Methods. The means of four experiments ± SD are given. FXIa in dilution buffer (○), in FXI-deficient plasma (•), or in EDTA plasma (□). Control EDTA plasma without FXIa (▵).