This assay employs the competitive inhibition enzyme immunoassay technique. An antibody specific for Vitamin A (VA) has  been  pre-coated  onto  a  microplate.  A  competitive  inhibition  reaction  is  launched  between Horseradish Peroxidase (HRP) labeled Vitamin A (VA) and unlabeled Vitamin A (VA) (Standards or samples) with the pre-coated antibody specific for Vitamin A (VA). After incubation the unbound conjugate is washed off. The amount of bound HRP conjugate is reverse proportional to the concentration of  Vitamin A (VA) in the sample. After addition of the substrate solution, the intensity of color developed is reverse proportional to the concentration of  Vitamin A (VA) in the sample.