Radioimmunoassay (RIA) is a scientific method used to test antigens (for example, hormone levels in the blood) without the need to use a bioassay. It was developed by Rosalyn Yalow and Solomon Aaron Berson in the 1950s. [Yalow RS, Berson SA. Immunoassay of endogenous plasma insulin in man. "J Clin Invest" 1960;39:1157-75. PMID 13846364.] In 1977, Rosalyn Sussman Yalow received the Nobel Prize in Medicine for the development of the RIA for insulin: the precise measurement of minute amounts of such a hormone was considered a breakthrough in endocrinology.

Although the RIA technique is extremely sensitive and extremely specific, it requires a sophisticated apparatus and is costly. It also requires special precautions, since radioactive substances are used. Therefore, today it has been largely supplanted by the ELISA method, where the antigen-antibody reaction is measured using colorometric signals instead of a radioactive signal.

To perform a radioimmunoassay, a known quantity of an antigen is made radioactive, frequently by labeling it with gamma-radioactive isotopes of iodine attached to tyrosine. This radiolabeled antigen is then mixed with a known amount of antibody for that antigen, and as a result, the two chemically bind to one another. Then, a sample of serum from a patient containing an unknown quantity of that same antigen is added. This causes the unlabeled (or "cold") antigen from the serum to compete with the radiolabeled antigen for antibody binding sites.

As the concentration of "cold" antigen is increased, more of it binds to the antibody, displacing the radiolabeled variant, and reducing the ratio of antibody-bound radiolabeled antigen to free radiolabeled antigen. The bound antigens are then separated from the unbound ones, and the radioactivity of the free antigen remaining in the supernatant is measured. A binding curve can then be plotted, and the exact amount of antigen in the patient's serum can be determined.

With this technique, separating bound from unbound antigen is crucial. Initially, the method of separation employed was the use of a second "anti-antibody" directed against the first for precipitation and centrifugation. The use of charcoal suspension for precipitation was extended but replaced later by Drs. Werner and Acebedo at Columbia University for RIA of T3 and T4. [cite journal |author=Werner SC, Acebedo G, Radichevich I |title=Rapid radioimmunoassay for both T4 and T3 in the same sample of human serum |journal=J. Clin. Endocrinol. Metab. |volume=38 |issue=3 |pages=493–5 |year=1974 |pmid=4815178 |doi=] An ultramicro RIA for human TSH was published in BBRC (1975) by Drs. Acebedo, Hayek et al. [cite journal |author=Acebedo G, Hayek A, Klegerman M, Crolla L, Bermes E, Brooks M |title=A rapid ultramicro radioimmunoassay for human thyrotropin |journal=Biochem. Biophys. Res. Commun. |volume=65 |issue=2 |pages=449–56 |year=1975 |pmid=1148002 |doi=10.1016/S0006-291X(75)80168-9]


External links

* [ Radioimmunoassay- procedure]

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