What are the similarities between ELISA and RIA (radioimmunoassay)?

ELISA (enzyme-linked immunosorbent assay) and RIA (radioimmunoassay) are two procedures based on immunoassay techniques. 

Both assays rely on antibody-antigen complex formation and both are highly sensitive and specific. Both techniques can be used to detect and quantify unknown proteins in a given sample and both are used in laboratories to diagnose diseases for various organisms.