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 drug or metabolite in the sample and drug or metabolite labeled with 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 by product is formed when it reacts with the drug or metabolite labeled with enzyme that has bound to the immobilized antibody. The absorbance reading on the spectrometer is inversely proportional to the concentration of drug or metabolite in the urine sample.