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. 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.