- Radioimmunoassay (RIA) is a highly sensitive and specific technique used to measure minute concentrations of antigens—such as hormones, drugs, or proteins—in biological fluids. First developed in the 1950s by Rosalyn Yalow and Solomon Berson, RIA revolutionized biomedical research and diagnostics by introducing a reliable method for quantifying substances at picomolar to femtomolar levels. The technique is based on the principles of antigen-antibody binding and the use of radiolabeled substances.
- In a typical RIA, a known quantity of an antigen is labeled with a radioactive isotope, commonly Iodine-125 (¹²⁵I). This radiolabeled antigen competes with the non-labeled antigen (from the test sample) for a limited number of binding sites on a specific antibody. The reaction reaches equilibrium, after which the bound and free antigens are separated—commonly by precipitation, adsorption, or using a solid-phase antibody. The radioactivity of either the bound or free fraction is then measured using a gamma counter. The amount of radioactivity detected is inversely proportional to the concentration of the unlabeled antigen in the sample.
- RIA is exceptionally sensitive and specific due to the high affinity and selectivity of the antibody-antigen interaction and the precise quantification enabled by radiodetection. It has been widely used for measuring hormones (e.g., insulin, cortisol, thyroid hormones), vitamins, drugs, and tumor markers in clinical and research laboratories.
- Despite its advantages, RIA has some limitations. The use of radioactive materials poses health, safety, and environmental concerns, requiring strict regulatory compliance, specialized facilities, and trained personnel. Moreover, the shelf-life of radiolabeled reagents is limited due to radioactive decay.
- With the advent of non-radioactive immunoassays such as enzyme-linked immunosorbent assays (ELISAs), chemiluminescence, and fluorescence-based methods, the routine use of RIA has declined. However, it remains a gold standard for some applications where ultrasensitive detection is essential and alternative techniques are less effective.
- In summary, radioimmunoassay is a landmark technique in analytical biochemistry and clinical diagnostics. It has enabled the precise measurement of biologically important molecules at extremely low concentrations, significantly advancing endocrinology, pharmacology, and disease monitoring.
