[Enzyme-immunoassay. New techniques for the photometric laboratory (author's transl)].

Enzyme-immunoassays (EIA) are a further development of the radioimmunoassay (RIA) employing enzymes instead of radionuclides as labels. This advance makes it possible also for the photometric laboratory to determine substances at extremely low concentrations. Heterogeneous EIA, mainly named ELISA (enzyme linked immunsorbent assay), are based on the same reaction principles as the analogous RIA. A characteristic feature is the bound-free separation, which is effected primarily by means of solid-phase techniques. The most sophisticated of these -- the solid-phase tube technique -- lets the immunological reaction take place on the inside wall of the reaction vial; the separation step is accomplished by simply emptying the vial. Heterogeneous EIA have a universal range of applications, from the determination of small haptens to the determination of macromolecular particles. Homogeneous EIA have so far been used only for the determination of haptenes. They require no bound-free separation step. The change in enzyme activity as a result of the enzyme-haptene-antibody reaction is proportional to the concentration of the haptene. In homogeneous assay systems, the photometric measurement is susceptible to interference by constituents of the sample material, whereas heterogeneous methods do not suffer from this disadvantage because the indicator reaction proceeds in an aqueous medium after the separation step.