Radioimmunoassay

Radioimmunoassay. It is a highly sensitive in vitro analytical technique used to measure minute concentrations of antigens, such as hormones, drugs, and vitamins, in biological fluids. The method combines the specificity of immunological reactions with the sensitivity of radioisotope detection, utilizing radiolabeled molecules. Its development revolutionized endocrinology and clinical chemistry, enabling precise quantification of substances previously undetectable.

Principle and mechanism

The fundamental principle relies on competitive binding between a radiolabeled antigen and an unlabeled antigen for a limited number of specific antibody binding sites. A known quantity of the antigen is labeled with a gamma ray-emitting radioisotope, commonly Iodine-125. This labeled antigen is mixed with a sample containing an unknown quantity of the same, unlabeled antigen and a controlled amount of specific antibody. Following an incubation period, the bound antigen-antibody complexes are separated from the free antigen. The radioactivity in either the bound or free fraction is measured using a gamma counter or scintillation counter. The concentration of the unknown antigen is determined by comparing its signal to a standard curve generated with known concentrations.

Development and history

The technique was pioneered in the late 1950s by American medical physicists Rosalyn Sussman Yalow and Solomon Berson at the Veterans Administration Hospital in the Bronx. Their initial work focused on measuring insulin levels in human plasma, a breakthrough documented in the Journal of Clinical Investigation. This discovery provided the first method to detect peptide hormones in blood and challenged prevailing assumptions in diabetes research. For this seminal contribution, Yalow was awarded the Nobel Prize in Physiology or Medicine in 1977, sharing it with Roger Guillemin and Andrew Schally; Berson had died and was thus ineligible for the award. The work established the field of radioimmunology.

Procedure and methodology

A standard procedure begins with the preparation of reagents, including the specific antibody, often produced in rabbits or guinea pigs, and the purified antigen labeled with Iodine-125 or Tritium. A series of standard solutions with known antigen concentrations are prepared to create a calibration curve. The unknown sample, standards, labeled antigen, and antibody are combined and incubated, typically at 4 °C, to allow the competitive reaction to reach equilibrium. Separation of bound from free antigen is then achieved using techniques such as precipitation with ammonium sulfate, polyethylene glycol, or a second antibody directed against the first, a method known as a double-antibody technique. The radioactivity of the isolated fraction is counted, and the unknown concentration is interpolated from the standard curve.

Applications and uses

It found immediate and widespread application in endocrinology for measuring hormones like growth hormone, thyroid-stimulating hormone, cortisol, and testosterone. It became crucial in therapeutic drug monitoring for agents such as digoxin and cyclosporine, and in screening blood donations for hepatitis B surface antigen. The technique was instrumental in reproductive medicine for measuring human chorionic gonadotropin in early pregnancy tests and follicle-stimulating hormone. It also advanced research in neuroscience by enabling the study of neurotransmitters and in oncology for detecting tumor markers like carcinoembryonic antigen.

Advantages and limitations

Its primary advantage is exceptional sensitivity and specificity, capable of detecting picogram to nanogram quantities per milliliter. It also offers high precision and the ability to process many samples simultaneously. However, significant limitations include the handling and disposal requirements for radioactive materials, governed by agencies like the Nuclear Regulatory Commission, which necessitates specialized facilities and safety protocols. The shelf-life of radiolabeled reagents is limited by radioactive decay, and the technique generally requires longer assay times compared to non-isotopic methods. Furthermore, it cannot easily distinguish between biologically active and inactive molecular forms.

Variations and related techniques

To address some limitations, several related techniques were developed. The immunoradiometric assay is a non-competitive variation where the antibody itself is radiolabeled. The enzyme-linked immunosorbent assay, which uses an enzyme label instead of a radioisotope, largely superseded it in routine clinical use due to safety and stability. Other related methods include the radioimmunosorbent test and the use of different labels like chemiluminescent compounds. The fundamental competitive binding principle also influenced the development of non-immunoassay techniques like radio receptor assays. Category:Laboratory techniques Category:Immunology Category:Nuclear medicine