Labeled antigen radioimmunoassays radioactivity measurement

Radioimmunoassay (RIA) These assays rely on the use of radiolabels. Typically, antigen is labelled using a radioactive isotope, for example iodine-125 ( 1), and radioactivity (radiation emitted from tube) is measured using an appropriate counter (in this case a y radiation counter). This major technique and some applications are covered in more detail later. 

Radioimmunoassay (RIA). RIA is a procedure used to detect or determine an antigen in a sample based on immunology. A radioisotope-labelled antigen is mixed into a sample, an antigen specific antibody is added to the mixture, and the amount of antibody bound with radiolabeled antigen is then measured by scintillation counter. The amount of antigen in the sample is calculated from the radioactivity. RIA is more sensitive than ELISA, but ELISA has replaced RIA because the latter requires the use of radioactive compounds. 

Determination of the distribution of Ag between the antibody-bound and free fractions, in radioimmunoassay procedures, requires a separation step which isolates Ag from Ag -Ab. Once separated, measurement of the radioactivity of the label in one or both the fractions provides a signal that is approximately exponentially related to the concentration of unlabeled antigen. The graphical relationship can have either a positive or a negative slope depending on whether the antibody-bound or free fraction is measured. The exact shape of the calibration curve however, is dependent on the antibody-binding equilibrium constant. ... 

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