Challenges with measuring tissue factor antigen and activity in human plasma.

Tissue factor (TF) is a transmembrane protein that, in association with its ligand factor VII (FVII)/activated factor VII (FVIIa), activates blood coagulation. TF is highly procoagulant and even very small amounts can activate blood coagulation. Levels of TF-positive extracellular vesicles (EVs) are increased in blood in diseases associated with thrombosis. However, it is challenging to accurately quantify the very low levels of TF in blood. Activity-based assays have higher sensitivity and specificity than antigen-based assays. Many anti-human TF antibodies have been generated but they differ in their affinity for TF and bind to different epitopes. They can be divided into 2 groups: those that compete with FVII/FVIIa binding to TF, and those that bind to both TF and the TF-FVII/VIIa complex. Commercial enzyme-linked immunosorbent assays are commonly used to measure TF antigen in plasma but have low sensitivity and specificity for the detection of TF antigen in plasma. Flow cytometry is used to measure TF antigen on EVs but also has low sensitivity and specificity. Functional TF activity assays should be performed in the presence and absence of an inhibitory anti-TF antibody to distinguish between TF-dependent and TF-independent FXa generation because FVIIa can activate FX in the absence of TF. TF pathway inhibitor inhibits the TF-FVIIa complex and reduces TF activity of isolated EVs. Two commercial assays are available for the measurement of TF activity of EVs isolated from human plasma. Measurement of TF activity of EVs isolated from plasma may be a useful biomarker of thrombotic risk in different diseases.