radioimmunoassayEnglishOxford Dictionary of Englishradioimmunoassay /ˌreɪdiəʊˌɪmjʊnəʊˈaseɪ , ˌreɪdiəʊɪˌmjuːnəʊˈaseɪ / ▸ noun Medicine a technique for determining antibody levels by introducing an antigen labelled with a radioisotope and measuring the subsequent radioactivity of the antibody component.