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Ria --> radioimmunoassay

(Science: investigation, technique) Any system for testing antigen antibody reactions in which use is made of radioactive labelling of antigen or antibody to detect the extent of the reaction.

More accurately it is a sensative method that is used to detect and quantitate any antigenic substance (protein maybe even some drugs) within a sample and depends on the amount of target antigen in that sample. The unknown sample is treated to a known amount of radioactively labeled antigen (same antigenic properties as the unknown amount of target antigen)and a known amount of the appropriate antibody. Competition between the known(labeled) and unknown (unlabeled) antigen for the limited amount of antibodies gives a measure of the amount of unknown target antigen in the sample by comparison to a previously constructed standard curve, because the quantity of isotope labeled antigen complexing with the antibody varies inversely with the quantity of unlabeled antigen.