RIA: Radioimmunoassay

A radioimmunoassay (RIA) is an immunoassay that uses radiolabeled molecules in a stepwise formation of immune complexes. A RIA is a very sensitive in vitro assay technique used to measure concentrations of substances, usually measuring antigen concentrations (for example, hormone levels in blood) by use of antibodies.

2. Principle of Radioimmunoassay (RIA)

immunoassay uses radiolsotope-labeled purified 20 which competes with
(unknown) antigen for binding sites on a known amount of antibody

The Ag-Ab complexes that form between the antigen and antibody can then be precipitated using the

second antibody and the amount of radioactivity of the bound complex is measured by means of

isotope analyzers and autoradiography.

‘When the sample contains a high amount of antigen, much ofthe antigen-binding sites of the antibodies

are occupied by unlabeled antigen. So the bound complex will show lite radioactivity. The

radioactivity generated, in fact, is inversely proportional to the amount of antigen present in the sample.

Then the concentration of the unknown (unlabeled) antigen or hapten present in a sample is determined

by plotting the value of radioactivity generated in a standard chart generated using different

concentrations of same antigens against he radioactivity of the bound complex

The test can be used 10 determine very small quantities (e. nanogram) of antigens and antibodies in
the serum.

The testis used for quantitation of hormones, drugs, HBsAg, and other viral antige

Analyze nanomolar 1 huis,

LIMITATIONS)

‘The handling of radioisotopes requires specific safety measures, Because of this limitation, RIA has
been replaced by ELISA in clinical laboratories,