Most isotope diagnostic methods for concentration measurement belong to the family of “protein binding assays.” Generally, they have the following main components:

• The specimen from which the concentration of a constituent (L, “ligand,” “analyte,” or “antigen”) is to be measured.

• A substance (e. g., antibody, Ab) specifically binding the ligand (L).

• A tracer, which can be either:

• The labeled version of the ligand (L*, in case of competitive assays) or

• A second, labeled antibody (Ab*, in excess amount) against the ligand to be measured.

• A method to separate the bound and free tracers.

In immunoassays, the specific binding material is a monoclonal or polyclonal antibody. Labeling may use a radioisotope, enzyme, chemiluminescent, or fluores­cent tracer.

Immune binding reactions are greatly influenced by a number of conditions (temperature, reaction times, pH, etc.); therefore, standards with known concentra­tions of the analyte are included in each series of measurements. A calibration curve based on these standards is used to calculate the concentration from the signal measured from an unknown sample.

In practice, various combinations of tracers, labeled components, competitive or sequential reactions, limited or excess amounts of components, and separation methods are used. The basic principles of the two most widespread methods are summarized next.