Radioimmunoassay (RIA) is a sensitive laboratory technique used to measure the concentration of antigens in a sample. It involves the use of radioactively labeled antibodies that bind to specific antigens, allowing for the detection and quantification of these substances. RIA is commonly used in clinical laboratories to assess hormone levels, drug concentrations, and other biomolecules. The process typically involves mixing a sample with a known quantity of radioactively labeled antigen and specific antibodies. The bound and free antigens are then separated, and the radioactivity of the bound fraction is measured. This allows for the determination of the original antigen concentration in the sample.