Both versions of this assay are exquisitely sensitive (usually about 1-50 ng/ml), economical in that very small volumes of sample and test materials are used, and convenient in that many tests can be done at once. It is one of the two most widely used laboratory tests for antigen or antibody.

The presence of either specific antibody or antigen in serum can be detected. If antibody is being determined, its antigen is bound to a small plastic tube. The non-bound antigen is removed by washing, and the serum containing the antibody is added and allowed to incubate for 1-24 hours (Animation above).

The nonbound antibody is removed by thorough washing, the radiolabeled antiimmunoglobulin (usually antibody made in a rabbit or goat to human immunoglobulin) is added and allowed to react. Again, the nonbound antibody is removed by washing, then the radioactivity retained in the tube is counted.

If antigen is to be determined, antibody specific for that antigen is coupled to the tube, and the serum added. Non-bound antigen is removed by washing, and radiolabeled antibody to the antigen is added. Radioactivity retained after washing is determined.

The original version of the RIA used this procedure except, instead of using radiolabeled antibody in the last step, radiolabeled antigen is added and allowed to compete with unlabeled antigen in the sample for binding to the tube-bound antibody. Thus, the more unlabeled antigen present, the lower the retained radioactivity.