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Glossary

RIA (Radioimmunoassay)

Radioimmunoassay is a competitive test using an I-125 tracer, invented by Yalow and Berson. The principle, and why it keeps a niche for very small peptides.

Radioimmunoassay (RIA) is a competitive immunoassay: a radiolabelled tracer antigen — in practice nearly always a peptide labelled with I-125 — and the unlabelled target from the specimen compete for a limited quantity of antibody. Once antibody-bound tracer has been split from free tracer, 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

RIA was developed by Solomon Berson and Rosalyn Yalow for insulin measurement — Yalow shared the Nobel Prize for it in 1977 — and no earlier technique had been 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 concern over I-125 decaying with a half-life of about 60 days. RIA remains where it is still superior: tiny peptides and steroid hormones that must be detected down to a few picograms, 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 label that loses activity at a fixed rate, and waste-disposal duties. It is usually these costs, rather than assay performance, that settle the decision.

See also antiserum, immunogen and species reactivity. The shop lists assay kits and tagged peptides; our guide explains how RIA and ELISA kits measure peptide levels.

All glossary terms

Further reading

Guides that reference this term

Molecule guides

Cardiovascular Research Peptides: Families, Reagent Formats and Selection Notes

Two opposing hormone systems define this catalogue section. Here is what sits in it, which format answers which question, and how the handling differs.

Molecule guides

Diabetes and Metabolic Research: Reagent Families, Antisera and Immunoassay Kits

C-peptide and insulin, GIP and GLP-1, glucagon, adipokines: how this section is arranged and which reagent format answers which experimental question.

Molecule guides

Peptide Antibodies and Antisera: Formats, Immunogens and Validation Controls

What a listing marked "antibody" really contains, why what it was raised against tells you most, and which controls let you stand behind a result.

Molecule guides

Peptide Immunoassay Kits: ELISA, EIA and RIA Formats and How to Select One

A kit is a packaged, validated procedure — but only for the matrix, species and analyte it was built around. What to check before you buy, and how to run it well.