The key difference between radioimmunoassay and immunoradiometric assay is that in radioimmunoassay, the sample or compound to be measured is combined with a radioactive antigen before the combination, while in the immunoradiometric assay, the sample or compound immediately combines with the radiolabeled antibodies.
An immunoassay is a biochemical test that detects the presence or concentration of macromolecules in a solution using an antibody or antigen. Fluorescent and radioactive antibodies are used to measure antigens in a sample. Initially, antibodies were used in precipitation techniques such as immunoprecipitation, immunodiffusion and immune-electrophoresis for serum protein analysis. At present, highly sensitive techniques such as radioimmunoassay and immunoradiometric assays are used for the measurement of drugs, tumor markers, and hormones.
CONTENTS
1. Overview and Key Difference
2. What is Radioimmunoassay Assay
3. What is Immunoradiometric Assay
4. Similarities – Radioimmunoassay and Immunoradiometric Assay
5. Radioimmunoassay vs Immunoradiometric Assay in Tabular Form
6. Summary – Radioimmunoassay vs Immunoradiometric Assay
What is Radioimmunoassay Assay?
Radioimmunoassay (RIA) is an immunoassay that uses radioactive elements stepwise formation of antigen-antibody complexes. RIA usually use radioactive antibodies to detect the amount of antigen in a sample. RIA is a very specific and highly sensitive in vitro assay. The principle behind RIA is competitive binding. Here, a radioactive antigen competes against a non-radioactive antigen for a constant amount of antibody or receptor binding sites. RIA requires special licensing and precautions since radioactive substances are used, and it remains among the least expensive techniques.
Figure 01: Immunoassay
During RIA, a known amount of antigen is made radioactive frequently by labelling it with gamma-radioactive isotopes of iodine attached to tyrosine. Then this antigen is combined with a known amount of antibody. Here, both antigen and antibody specifically bind to one another. Then, a blood-serum sample is added to initiate a competitive reaction between the labeled antigens and unlabeled antigens in the serum with specific antibodies. In this reaction, antibodies release a certain amount of labeled antigen. This quantity is proportional to the ratio of labeled to unlabeled antigen. Finally, a binding curve is generated to derive the amount of antigen in a patient’s blood serum.
What is Immunoradiometric Assay?
Immunoradiometric assay (IRMA) is an immunoassay that uses radiolabeled antibodies. In IRMA, the antibodies are labelled using radioisotopes. These antibodies bind to the antigens present in a specific sample. In a positive sample, antibodies that are radioactively labeled bind to the free epitopes of antigens. This forms an antigen-antibody complex.
During the second reaction, the labeled antibodies that are unbound are removed using a solid phase antigen. The remaining number of radioactive antibodies in the solution is a direct function of the antigen concentration. IRMA is known as an excess reagent assay where an excess amount of radiolabeled antibody is used as the reagent. Here, an excess concentration of labeled antibody or antigen is allowed to react. As the final step, the antigen-bound and free antibodies are separated, and the antigen bound fraction is subjected to radioactive assays. Here, the activity of the fraction is directly proportional to the concentration of the antigen.
What are the Similarities Between Radioimmunoassay and Immunoradiometric Assay?
- Both radioimmunoassay and immunoradiometric assay are immunoassays that use radioactive elements in the stepwise formation of antigen-antibody complexes.
- They form an antigen-antibody complex.
What is the Difference Between Radioimmunoassay and Immunoradiometric Assay?
Radioimmunoassay is an immunoassay that determines antibody levels by an antigen labeled with a radioisotope while immunoradiometric assay is an excess reagent assay that uses an excess concentration of radiolabeled antibody. Thus, this is the key difference between radioimmunoassay and immunoradiometric assay. IRMA is capable of providing higher sensitivity than RIA. In RIA, antigens are labeled with gamma-radioactive isotopes of iodine while in IRMA antibodies, are labeled using isotopes of iodine. So, this is also a difference between radioimmunoassay and immunoradiometric assay. Since IRMA is an excess reagent technique, the assay is performed in a shorter time than RIA.
The following table summarizes the differences between radioimmunoassay and immunoradiometric assay.
Summary – Radioimmunoassay vs Immunoradiometric Assay
Radioimmunoassay is an immunoassay that use radioactive elements in stepwise formation to determine antibody levels. RIA usually uses radioactive antibodies. A radioactive antigen competes against a non-radioactive antigen for a constant amount of antibody or receptor binding sites. The immunoradiometric assay is an immunoassay carried out to determine antigen levels of a sample using radiolabeled antibodies. These antibodies bind to antigens present in a specific sample. At the end of each assay, an antigen-antibody complex is formed. In order to obtain the results of the assays, a binding curve is drawn. In radioimmunoassay, the labeled antigen quantity is proportional to the ratio of labeled to unlabeled antigen, but in an immunoradiometric assay, the activity of the antigen bound fraction is directly proportional to the concentration of the antigen. This is the summary of the difference between radioimmunoassay and immunoradiometric assay.
Reference:
1. “Radioimmunoassays (RIAs).” Perkin Elmer.
2. WOODHEAD, J. S., ADDISON, G. M., HALES, C. N. (1974). The immunoradiometric assay and related techniques. British Medical Bulletin, 30(1), 44-49. doi:10.1093/oxfordjournals.bmb.a071166
Image Courtesy:
1. “Immunoassay” By Gringer – Own work (CC0) via Commons Wikimedia