Radioimmunoassay

Radioimmunoassay is a highly sensitive immunoassay technique used to measure small amounts of antigen in a sample by using antibodies and radiolabeled tracers. The method is based on the specific interaction between an antigen and its antibody and relies on a competition between labeled and unlabeled antigen for a limited amount of antibody.

In a typical radioimmunoassay, a known amount of radiolabeled antigen competes with the unlabeled antigen in the sample for binding to a limited antibody. After incubation, bound and free antigens are separated, and the radioactivity of the bound fraction is measured with a gamma counter or similar detector. The amount of labeled antigen bound to antibody is inversely related to the concentration of unlabeled antigen in the sample, allowing quantification by comparison to a standard curve. Isotopes commonly used include Iodine-125 and tritium, among others. Assays can be performed as heterogeneous formats requiring a separation step or as homogeneous formats with direct measurement.

Radioimmunoassay was developed in the 1950s by Solomon Berson and Rosalyn Yalow, whose work earned the Nobel Prize in Physiology or Medicine in 1977. The technique offers very high sensitivity and a broad dynamic range and requires relatively small sample volumes. Limitations include the use of radioactivity, safety and regulatory concerns, waste disposal, and the need for specialized facilities and trained personnel. RIA has largely been supplanted in many applications by non-radioactive immunoassays, but it remains in use for certain hormones, drugs, and research contexts where extreme sensitivity is required.