(RIA) A radioimmunoassay is an in vitro assay test in which very small quantities of hormones, drugs and other substances in blood, urine, etc. can be measured by using specific antibodies or antigens which have been labeled with radioactive tracers. Since the patient does not receive the radioactive material, there is no patient radiation exposure.
Radioimmunoassays are usually replaced by fluoroimmunoassay and enzymimmunoassay to avoid the use of radioactive materials.

See also Bioassay, Fluorescent Immunoassay, Protein Binding Assay, Radioassay, and Berson Salomon.