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Radioimmunoassay

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  p384-385

2026-11-04

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Radioimmunoassay

RIA was the forerunner of heterogeneous immunoassays. Difficulties associated with the handling and storage of radioactivity; disposal of radioactive waste and the half-life of the radioactive labels have resulted in RIA being replaced largely by non-isotopic EIA methods. Two main types of RIA have been used for drug testing based on the isotope employed. An isotopic drug label is termed a tracer. Use of tritium (3H)- or 14C-labelled drug allows an identical molecular structure of tracer to the drug being tested, although counting the beta emissions involves an organic-based scintillation fluid and a sophisticated beta scintillation counter. Use of the gamma-emitting 125I as tracer allows faster and more efficient counting and a number of research (Hand et al. 1986) and commercial RIAs were based on this isotope. The immunoassay principle for RIA is the same as that for EIA described above. Antibody coated tubes have been used to good effect with iodinated tracers. Simple decanting easily separates the bound and free fractions, and allows quantification of the bound fraction using a gamma counter. Other ways to separate bound and free fractions include the use of a second antibody precipitation step. This requires a centrifuge to form a pellet of bound material that can be counted after the free fraction has been decanted or aspirated. Magnetic beads and other particles coated with a second antibody have also been used to aid this separation of the bound and free fractions. Dextran-coated char coal has been used to absorb free tracer from solution, and thereby allow the antibody-bound fraction in the supernatant to be counted.