Radioimmunoassay

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A radioimmunoassay ( RIA , English : Radioimmunoassay ) is a laboratory method for the quantitative determination of the smallest amounts of substances. The smallest concentrations (sometimes pg / ml) of hormones , enzymes , tumor antigens , infection antigens , drugs and DNA can be reliably and specifically determined using these radioimmunological methods. The RIA was the first immunassay procedure , later followed by the IRMA. The iodine isotope 125 I is often used for radioactive labeling due to its favorable disintegration properties ( γ-emitter , HWZ 59.6 days) . Due to the higher technical complexity with an isotope laboratory, the ELISA as well as several other non-radioactive methods such as FPIA, MEIA, ECLIA etc. are increasingly used today . These work according to the same basic principles as RIA and IRMA.

The prerequisite for RIA and IRMA is the availability of specific antibodies against the antigens to be determined . These antibodies are obtained by immunizing animals or as monoclonal antibodies from cell cultures.

Test variant RIA

The antigen to be measured (i.e. the sample) is reacted (“incubated”) with the specific antibody along with a known amount of radiolabelled antigen (the tracer). The (natural) antigen to be measured and the radioactively marked (artificial) antigen bind competitively to the antibody; they have the same binding behavior to the antibody. The binding sites are limited by the concentration of the antibody; the more natural antigen is present, the less radioactive antigen is bound. At the end of the incubation period, unbound antigen is washed away and the bound radioactivity is measured. Then the antigen concentration in the sample can be calculated back.

Test variant IRMA

Another in vitro nuclear medicine test method is the immunoradiometric assay ( IRMA ). In this test, it is not an artificial version of the substance you are looking for that is radioactively marked ( tracer ), but a second antibody that is directed against another binding site of the antigen. Therefore, IRMAs are only possible with antigens whose molecules are large enough that two antibodies can bind to the antigen without interfering with one another.

The advantage of the IRMA lies in the greater accuracy at very low concentrations of the substance to be determined, a disadvantage in false-low results at very high concentrations of the antigen (“high-dose hook effect)”.

The developers of the RIA method

The radioimmunoassay method was first tested in practice by Solomon Aaron Berson and Rosalyn Yalow in 1959 with the determination of insulin . Yalow received the Nobel Prize in Physiology or Medicine for this in 1977 .

literature

  • Reingard Senekowitsch-Schmidtke: Immunoassays, quality control . In: Torsten Kuwert u. a. (Ed.) Nuclear Medicine . Thieme Verlag, Stuttgart 2008, ISBN 978-3-13-118504-4 .