This kit uses the Competitive-ELISA principle. The microtiter plate strips has been pre-coated with Coating antibody for DA. Standards or samples containing DA and biotin labeled DA are added to the plate, A competitive inhibition reaction is launched between labeled DA and free DA (Standards or samples) with the solid phase antibody specific to DA. After that, Streptavidin-Horseradish Peroxidase(SA-HRP) is added to form a sandwich complex of solid phase antibody-biotin labeled antigen-SA-HRP. And then, TMB substrate solution is added to all wells and incubated. An enzyme-catalyzed reaction generates a blue color in the solution, thereafter, stop solution is added to stop the substrate reaction and the color turns yellow. The yellow solution is read at a wavelength of 450nm. The concentration of DA in the samples is then calculated from the OD value by establishing a standard curve.