In enzyme-linked immunosorbent assays (ELISA), the antibody is anchored to a solid surface. The patient sample, as well as a drug or metabolite labeled with enzyme, are added to a testing well where the drug or metabolite in the sample and drug or metabolite labeled with an enzyme compete for binding sites on the anchored antibody. Unbound components are removed by washing the well and a chromogenic reagent is added. A colored product is formed when the chromogenic reagent reacts with the immobilized drug labeled with an enzyme. The absorbance reading on the spectrometer is inversely proportional to the concentration of drug or metabolite in the urine sample.
