A Radioimmunoassay (RIA) test is a highly sensitive in vitro assay used to measure the concentration of substances, often antigens, in a sample, like blood or saliva. It utilizes radioactive isotopes to label antigens or antibodies, enabling detection of very small quantities (picograms) through competitive binding with unlabeled antigens. RIA revolutionized research and clinical practice by enabling the detection of hormones and other substances at previously undetectable levels