It differs from conventional radioimmunoassay (RIA) in that the compound to be measured combines immediately with the radiolabeled antibodies, rather than displacing another antigen by degrees over some period.

Immunoradiometric assay (IRMA) was first introduced by "Miles and Hales" in 1968, who proposed certain theoretical advantages of the method with regard to improving the sensitivity and precision of immunoassays.

In IRMA, the antibodies are labeled with radioisotopes which are used to bind antigens present in the specimen.

Unbound labeled antibodies are removed by a second reaction with a solid phase antigen.

The amount of radioactive remaining in the solution is direct function of the antigen concentration.