Measurement of tissue factor-positive extracellular vesicles in plasma: strengths and weaknesses of current methods

This review evaluates the different methods used to measure levels of tissue factor (TF) in plasma and on extracellular vesicles (EVs). Levels of TF-positive (TF+) EVs in blood are increased in a variety of diseases, such as cancer, sepsis, and viral infection, and are associated with thrombosis. Highly sensitive assays are required to measure the low levels of TF+ EVs in blood.

TF antigen levels in plasma have been measured using standard ELISAs, SimpleStep ELISA technology, and solid-phase proximity ligation assay. Some studies reported the detection of TF+ EVs in plasma by flow cytometry. In addition, TF+ EVs can be captured onto beads and chips using anti-TF antibodies. Several assays have been developed to measure TF activity in EVs isolated from plasma. Importantly, activity-based assays are more sensitive than antigen-based assays as a single TF/FVIIa complex can generate large amounts of FXa.

We recommend isolating EVs from plasma and measuring TF activity using a functional assay in the presence and absence of an anti-TF antibody. We do not recommend using antigen-based assays as these are not sensitive enough to detect the low levels of TF in plasma.