Principle of Assay The AssaySense Human Tissue Factor Chromogenic Activity Assay Kit is developed to determine human TF chromogenic activity in plasma, tissue, and cell culture supernatants. The assay measures the ability of lipoprotein TF/FVIIa to activate factor X (FX) to factor Xa. The amidolytic activity of the TF/FVIIa complex is quantitated by the amount of FXa produced using a highly specific FXa substrate releasing a yellow para-nitroaniline (pNA) chromophore. The change in absorbance of the pNA at 405 nm is directly proportional to the TF enzymatic activity.