Immunoassays are bioanalytical tests used to detect and quantify specific molecules, depending on the reaction of an antigen and an antibody. Analytes can be proteins, hormones, antibodies, and antigens in a complex biological sample like blood or saliva. Immunoassays are widely used in medical diagnostics, therapeutic drug monitoring, and clinical studies in drug discovery and pharmaceutical fields. For example, they are very valuable in the early stages of drug development, which requires high-throughput screening of a large number of samples.1
Their main advantages are their full automation, short turnaround time, high specificity and sensitivity.2 This is due to the highly specific nature of antigen-antibody interactions, which also allows them to detect very low concentrations of target molecules and minimize background noise. In addition to their versatility for a wide range of targets such as hormones, drugs, proteins, and infectious agents, immunoassays have a relatively inexpensive cost, making them indispensable for researchers worldwide.
Besides ELISA (enzyme-linked immunosorbent assay), one of the most common immunoassays, there are also radioimmunoassays, fluorescent immunoassays, chemiluminescent immunoassays, western blotting, and lateral flow assays available. Radioimmunoassays are the earliest form of immunoassay, using radioactive isotopes to label antigens or antibodies. The amount of radioactivity is then measured to determine the concentration of the target molecule.3 They are still used today, for instance, to measure thyroid hormones in the body, which cooperates with the growth hormone to regulate childhood development.4
Fluorescent immunoassays rely on fluorescent labels which are measured when exposed to a specific wavelength of light to indicate the amount of the target molecule. Researchers have used it to detect and quantify cardiac troponin I, which is a key element of muscle regulation and contraction, being the most specific biomarker for myocardial infarction (MI), also known as heart attacks.5 Detecting MI can also be carried out with chemiluminescent immunoassays, which detect and quantify emitted light. The light results from a binding event which leads to a chemical reaction that produces light.6
Despite all the benefits of using immunoassays, there are limitations to consider. For instance, cross-reactivity may occur if antibodies recognize and bind to molecules other than their intended targets, leading to false positive results. The presence of other substances in a sample may also interfere with the immunoassay, leading to inaccurate results. In healthcare, the limitations of immunoassays can be harmful, such as a lack of concordance, when the assay is performed with one set of reagents which does not measure the same concentration as another. Problems may also arise due to autoantibodies, if epitopes on the analyte of interest are blocked from reagent antibodies by endogenous immunoglobulins.7 Therefore, while immunoassays can be used to help diagnose human disease, continuous research should not be overlooked.
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References:
Darwish, I. A. (2006). Immunoassay Methods and their Applications in Pharmaceutical Analysis: Basic Methodology and Recent Advances. International Journal of Biomedical Science : IJBS, 2(3), 217-235. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3614608/
Wauthier, L., Plebani, M. & Favresse, J. (2022). Interferences in immunoassays: review and practical algorithm. Clinical Chemistry and Laboratory Medicine (CCLM), 60(6), 808-820. https://doi.org/10.1515/cclm-2021-1288
Wu, Z. (2022). Effect of Radioimmunoassay on Accuracy of Thyroid Hormone Detection. Contrast Media & Molecular Imaging, 2022. https://doi.org/10.1155/2022/9206079
Cinquanta, L., Fontana, D. E., & Bizzaro, N. (2017). Chemiluminescent immunoassay technology: What does it change in autoantibody detection? Auto-Immunity Highlights, 8(1). https://doi.org/10.1007/s13317-017-0097-2
Hoofnagle, A. N., & Wener, M. H. (2009). The Fundamental Flaws of Immunoassays and Potential Solutions Using Tandem Mass Spectrometry. Journal of Immunological Methods, 347(1-2), 3. https://doi.org/10.1016/j.jim.2009.06.003
