Practice question

Radioimmunoassay achieves exceptional sensitivity through detection of gamma emissions from iodine-125 labeled antigen or antibody. Radioactive decay counting by gamma counters can resolve extremely low picomolar to femtomolar analyte concentrations, surpassing typical enzyme chromogenic limits where product must accumulate to visible levels. High affinity antibodies combined with competitive displacement allow quantification of hormones, thyroid hormones and small metabolites. While Western blot, immunodiffusion and standard ELISA are valuable for larger amounts or qualitative confirmation, they require higher analyte mass and generate higher background, making RIA historically gold standard for trace quantification despite safety constraints.