Glossary
RIA (Radioimmunoassay)
RIA is a competitive immunoassay built on an I-125 tracer and developed by Yalow and Berson. How it works and why it survives for measuring small peptides.
Radioimmunoassay (RIA) is a competitive immunoassay: a radiolabelled tracer antigen — in practice nearly always a peptide labelled with I-125 — competes with the unlabelled analyte in the sample for a restricted amount of antibody. After bound and free fractions are separated, the radioactivity in the bound fraction is measured in a gamma counter. The more analyte present, the lower the signal, so the calibration curve runs in reverse.
Why RIA is still around
Rosalyn Yalow and Solomon Berson created RIA to measure insulin, work that brought Yalow a share of the 1977 Nobel Prize; it was the first method sensitive enough to quantify hormones in the circulation. Most labs have since moved to ELISA, which needs no radioactive licence, no radioactive waste handling and no worry about the roughly 60-day half-life of I-125. RIA remains where it is still superior: very small peptides and steroids for which sensitivity in the low picogram range is critical, and sample matrices in which enzymatic or coloured interferents distort an optical signal.
The practical hurdles are regulatory as well as technical — a licence for radioactive materials, dedicated counters, a tracer that decays on a fixed timetable, and waste-disposal duties. It is usually these costs, rather than assay performance, that settle the decision.
Related terms and material
immunogen · antiserum · species reactivity. Reference material: assay kits, labeled peptides. Further reading: how ELISA and RIA kits quantify peptides.