Effect of membrane fusion on TF-VIIa activity. (A) TF-bearing microvesicles were incubated with 100 μg/mL of either bovine serum albumin (BSA) or unlabeled annexin V, and then washed. Microvesicles (MVs) treated with annexin V exhibited no binding of FITC-conjugated annexin V above an EDTA-treated sample, indicating blockade of phosphatidylserine on their surfaces. (B) TF-VIIa activity was measured using a chromogenic assay based on Xa generation. These values are adjusted for the direct effect of annexin V on the chromogenic assay. (C) Annexin V–coated microvesicles alone had approximately 35% less TF-VIIa activity than an equal number of BSA-treated microvesicles. n = 4; *P < .05. Error bars indicate ± SD.