Validation Criteria for Radioimmunoassay

Abstract

From a pharmacologist’s point of view, radioimmunoassay (RIA) represents a highly sophisticated form of bioassay, substituting the classical smooth muscle strip with a soluble antibody as the biological reactant. Both assay methods involve indirectly assessing the concentration of the unknown by comparing its effects on a measurable read-out of the reaction evoked (changes in radioactivity, tension, optical density, etc.) with those of known concentrations of a standard. Many bioassay techniques involve measuring biological responses that are mediated by agonist—receptor interactions. Such interactions usually occur in accordance with the law of mass action, similarly to antigen—antibody reactions. The presence of a radioactive tracer in RIA greatly enhances the sensitivity of the detection by limiting the mass of the antigen involved, to an extent inversely related to its specific activity. The specificity is also potentially increased in RIA vis-à-vis bioassay by virtue of a smaller number of possible reagents being present in a highly diluted antiserum than in isolated cell preparations or whole tissue fragments. Obviously, biological activity of the unknown is inherently measured by the latter, but not necessarily by the former. Thus, what is measured by RIA is immunochemical behaviour which may or may not be related to parts of the molecule responsible for biological activity (Yalow 1982).