The main difference between RIA and IRMA is that RIA uses radioactively labeled molecules known as tracers and antibodies to measure the concentration of a specific substance in a sample, whereas IRMA uses solid phase immobilization of labeled molecules where the labeled molecules are attached to a solid support like a bead or a plate.
RIA and IRMA are two types of immunoassays. Immunoassays are a highly selective bioanalytical method that measures analyte concentration or availability by using an antigen or an antibody as a biochemical agent.
Key Areas Covered
1. What is an Immunoassay
– Definition, Features, Applications
2. What is RIA
– Definition, Features, Uses
3. What is IRMA
– Definition, Features, Uses
4. Difference Between RIA and IRMA
– Comparison of Key Differences
Key Terms
RIA, IRMA, Immunoassay, Radioimmunoassay, Immunoradiometric Assay
What is an Immunoassay
An immunoassay is a type of biochemical test that uses the specific binding of antibodies to antigens for the detection and quantification of various molecules in a sample. Radioactive antibodies and fluorescent antibodies are the antibodies that help measure the antigens in the sample. In addition, antibody and antigen reactions help immunoassays to detect the required substance’s concentration.
Applications of immunoassays are common in laboratory medicine, research, and hospitals. The results of these immunoassays help to identify illnesses, determine the severity of illnesses, as well as to treat illnesses. In scientific research, immunoassays help track different substances, such as proteins, hormones, and antibodies. Immunoassays are also useful in detecting food and water contaminants.
Furthermore, there are several types of immunoassays. Some examples are radioimmunoassay (RIA), fluoroimmunoassay (FIA), enzyme immunoassay (EIA), chemiluminescent immunoassay (CLIA), and counting immunoassay (CIA).
Figure 1: A Simple Diagram of Parts of an Immunoassay: Aanalyte (green), Immunoglobulin (black), and Detectable Label (yellow)
What is RIA
RIA is a type of immunoassay that uses radiolabeled molecules in a stepwise formation of immune complexes. RIA stands for radioimmunoassay. Two methods of RIA are common in drug detection. They are double antibody RIA and coated tube RIA. In double antibody RIA, adding a secondary antibody promotes the precipitation of the bound primary antibody. Here the bound labeled drug is removed once this antibody-antigen complex precipitates. In the coated tube RIA, the primary antibody is coated on the inside of the tube. The removal of the unbound labeled drug involves pouring a supernatant. These RIAs are also sensitive and specific. Therefore, they need special handling and disposal of radioactive waste.
The principle underlying radioimmunoassay is competitive binding. Here, the radioactive antigen competes with a non-radioactive antigen for a certain fixed number of receptor binding sites or antibody binding sites. Moreover, RIA uses only one type of antibody to detect the target substance.
Furthermore, there are many uses for this method of immunoassay. Some of these uses are early cancer detection, measurement of growth hormone levels, tracking of leukemia viruses, blood bank screening for hepatitis, narcotics detection, diagnosis and treatment of peptic ulcers, and research with neurotransmitters.
What is IRMA
IRMA is an immunoassay that uses radiolabeled antibodies. IRMA stands for immunoradiometric assay. Here, the compound for measurement combines with radiolabeled antibodies immediately. This method uses two types of antibodies – one to capture the target substance and the other to detect it. Moreover, IRMA is a non-competitive assay in which the analyte to be measured is sandwiched between two antibodies.
IRMA uses solid phase immobilization of labeled molecules where the labeled molecules are attached to a solid support such as a bead of a plate. The sensitivity of the IRMA method is high. This is because of the use of two antibodies and the immobilization of the tracer. IRMA is also more precise as it is less susceptible to interference from other substances in the sample. However, the IRMA method is more costly and difficult to perform because of the immobilization of the tracer.
The principle of the IRMA is as follows. Upon the addition of a positive sample to the tubes, the radioactively labeled antibodies bind to the free epitopes of the antigen. Then it forms an antigen-antibody complex. The unbound labeled antibodies are removed by another reaction with the solid phase antigen. Hence, the amount of radioactive substance remaining in the solution is a direct function of the antigen concentration.
Difference Between RIA and IRMA
Definition
RIA is an immunoassay that uses radioactively labeled molecules known as tracers and antibodies to measure the concentration of a specific substance in a sample, whereas IRMA is an immunoassay that uses solid phase immobilization of labeled molecules where the labeled molecules are attached to a solid support like a bead or a plate.
Type of Antibodies
RIA uses a single type of antibody, whereas IRMA uses two types of antibodies – one to capture the target substance and the other to detect it.
Sensitivity
The sensitivity of IRMA is higher than that of the RIA because of the use of two antibodies and the immobilization of the tracer.
Cost of Operation and Ease of Use
RIA is less costly and easier to perform than the IRMA as IRA does not need the immobilization of the tracer.
Conclusion
In brief, immunoassays are a highly selective bioanalytical method that measures analyte concentration or availability by using an antigen or an antibody as a biochemical agent. There are different types of immunoassays. RIA and IRMA are two methods of immunoassays. The main difference between RIA and IRMA is that RIA uses radioactively labeled molecules known as tracers and antibodies to measure the concentration of a specific substance in a sample, whereas IRMA uses solid phase immobilization of labeled molecules where the labeled molecules are attached to a solid support like a bead or a plate.
Reference:
1. “Immunoradiometric Assay.” Science Direct.
2. “Radioimmunoassay.” Science Direct.
Image Courtesy:
1. “Immunoassay” By Gringer – Own work (CC0) via Commons Wikimedia
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