Properties, variants, and applications of the immunoradiometric assay method

The immunoradiometric assay (IRMA) and 2-site IRMA techniques employ purified radioactive antibodies to convert soluble antigens into a directly detectable complex. Unreacted labelled antibody is discarded by reaction with solid-phase antigen (IRMA), or by a preliminary insolubilization of the unknown antigen using solid-phase antibody (2-site IRMA). The preparation and properties of antigen-immunoadsorbent, solid-phase antibody, and purified radioactive antibodies are discussed. Analysis of the dose-response curve and problems in dose-interpolation are similar in IRMA and 2-site IRMA, and different from radioimmunoassay and other competitive-binding assay systems. The most significant assay variables include: exchange of radioactive antibody in IRMA reaction 2, stability and reactivity of the solid-phase antibody, washing the solid-phase, non-specific interference by serum proteins, and a paradoxical fall in tube radioactivity at high dose (2-site IRMA) (the 'high-dose hook effect'). 2-site IRMA has advantages in specificity (by using separate antibodies directed to 2 different antigen sites), sensitivity and assay range (because of the low zero dose-response). Assay variants include: alternative antibody labels, various solid-phase antibody preparations (including immunological 'spacer-arms'), non-immunological insolubilization of antigen, univalent radioactive antibodies, repeated immunological extractions of unknown antigen, and non-immunological separations in IRMA reaction 2. Both IRMA and 2-site IRMA have been adapted to the use of labelled anti-IgG as an additional 'universal' reagent, thereby avoiding the necessity for the preparation of radioactive antibodies specific for the unknown antigen. These techniques have been applied to the assay of a great variety of antigens and will be especially appropriate for automation.