Evaluation of 3 commercial assays for the measurement of tissue factor activity in human plasma.

Background: Levels of tissue factor (TF)-positive extracellular vesicles (EVs) are increased in plasma from patients with different diseases. However, the low TF levels in plasma are difficult to measure due to the presence of TF pathway inhibitor and the low sensitivity of assays. Objectives: We evaluated the ability of 3 commercial kits to measure TF activity levels in human plasma. Methods: We used the Abcam, AssaySense, and Actichrome colorimetric assay kits to measure recombinant TF (Innovin) and procoagulant activity (PCA) in plasma from lipopolysaccharide (LPS)-stimulated human whole blood from 4 healthy donors. We used different anticoagulants and plasma preparations according to the recommendations of the different kits. We examined the effects of depleting EVs and adding an anti-TF antibody on PCA. The Chapel Hill EV TF activity assay was used for comparison. Results: All 3 kits detected Innovin. The commercial kits detected higher PCA levels in some LPS treated samples but not others compared to controls Higher PCA levels were detected with the Abcam and AssaySense kits compared with the Actichrome kit. Depletion of EVs abolished PCA in both control and LPS plasmas. The Abcam and AssaySense kits detected TF-dependent PCA in LPS samples, but with lower sensitivity compared with the Chapel Hill assay. Conclusion: In this study, we found that the Abcam and AssaySense kits detected TF-dependent PCA in human plasma. However, these assays had lower sensitivity and specificity compared with the Chapel Hill EV TF activity assay.