Is it farther or further?
radioimmunoassay ra·di·o·im·mu·no·as·say (rā'dē-ō-ĭm'yə-nō-ās'ā, -ĭ-myōō'-)
The immunoassay of a substance, such as a hormone or an enzyme that has been radiolabeled.
An immunoassay in which the substance to be identified or quantified is labelled with a radioactive substance (called a tracer), such as an ion. See also immunoassay, tracer.