The DIAsource CA 15-3-IRMA is a two-step immunoradiometric assay based on coated-tube separation. Mab1, the capture antibody, is attached to the lower and inner surface of the plastic tube. Add calibrators or samples to the tubes. After incubation, washing removes the occasional excess of antigen. Addition of Mab2, the signal antibody labelled with 125I, will complete the system and trigger the immunological reaction. After washing, the remaining radioactivity bound to the tube reflects the antigen concentration. .