The Factor Xa Inhibitor Screening Assay is a colorimetric assay designed to measure the activity of human Factor Xa for screening and profiling applications. The assay kit comes in a convenient 96-well format or 384-well format and contains enough purified human Factor Xa, a chromogenic substrate, and PR-02 buffer for 100 or 400 reactions.To determine the effect of an inhibitor on Factor Xa activity, the enzyme should be preincubated with or without the test inhibitor prior to adding the chromogenic substrate to the reaction. The assay was functionally validated using Rivaroxaban, a potent inhibitor of Factor Xa.Upon proteolysis, Factor Xa cleaves the chromogenic substrate at the C-terminal end releasing p-nitroanilid (pNA), which produces a yellow color that is measurable photometrically at λ=405 nm. The increase in color is proportional to Factor Xa activity.