In a RIA, the analyte is incubated in a buffer with the antibody and a known quantity of radiolabeled analyte. After incubating these reactants for a period, the samples are centrifuged and the radioactivity in the bound, pellet fraction is counted (in some cases, the unbound tracer in the supernatant is counted instead). As the amount of analyte increases, more radioactive analyte is displaced and the amount of radioactivity in the pellet decreases. Therefore, low radioactivity corresponds to higher amounts of actual analyte in the sample (see Fig. 3).

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