1.Antigen-specific antibody is attached to a solid-phase surface (plastic beads)
2.Test specimen is added, which may or may not contain the antigen
3.An enzyme-labeled antibody specific to the antigen is added (conjugate)
4.Chromogenic substrate is added, which in the presence of the enzyme, changes color. The amount of color that develops is proportional to the amount of antigen in the test specimen.