Radioimmunoassay (RIA) and enzyme-linked immunosorbent assay (ELISA) are sensitive in vitro techniques used to measure concentrations of substances like hormones. RIA uses radiolabeled molecules to form immune complexes, while ELISA uses an enzyme-linked antibody and color change reaction to detect the presence of an antigen. Both techniques involve competitive binding between labeled and unlabeled antigens or antibodies. ELISA is commonly used for medical diagnostics and quality control due to its specificity, sensitivity, and not requiring radioactive substances. It exists in direct, indirect, sandwich, and competitive formats to detect different types of antigens or ligands.