The homogenous assays are usually EIAs of the enzyme-multiplied immunoassay technique (EMIT) type. Their principle consists in either the blocking or enhancing of the activity of the enzyme that is used as a label (Fig. 22.9). The formation of the antigen-antibody complex can result in hindrance of the substrate-enzyme reaction. Homogenous assays are somewhat easier to perform and can easily be automated. They are, however, less sensitive than conventional immunoassays that require separation and can therefore be used for measuring substances of low molecular weight and which are present in the sample in relatively high concentrations, such as T4.
