1.2 The Period of Modern Immunoassay
The development of modern immunoassay benefited from the development of labeled immunological technology. The clinical problems, which classical immunoassay could not solve, can be solved by modern immunoassay. It mainly included fluorescent labeling technology, radioimmunoassay, enzyme-linked immunoassay (ELISA), chemiluminescence immunoassay, and solid-phase membrane immunoassay.
Fluorescent Labeling
The initial labeling used in labeled immunoassay technology was fluorescein. Currently, indirect immunofluorescence tests were used to detect bacteria, viruses, parasites, tissue cell antigens, tumor-specific antigens, and autoimmune disease markers. Automation was available for time-resolved fluorescence and fluorescence polarization immunoassay which have been widely applied in diagnosis of various diseases.
Radio-Immunoassay (RIA)
The radio-immunoassay which is highly sensitive at that time has solved the problem of detection for biological-active substances such as hormones that were previously difficult to measure accurately. Although radio-immunoassay was a milestone in the development of immunoassay, due to short half-life, difficulty of waste handling and environmental pollution, it has gradually withdrawn from its application in routine clinical application. The use of non-radioactive labeling has become the most chosen option.
Enzyme-Linked Immunoassay (ELISA)
After the appearance of this simple and convenient technology, the enzyme-linked immuno-sorbent assay (ELISA) not only became a very simple tool for research but was also quickly applied to the clinical detection of various biological substances and markers, which is replacing RIA step by step. Various techniques have been invented one after another, such as one-step sandwich method, different homogeneous enzyme-labeled or radionuclide-labeled methods, and so on. However, people were still seeking new methods to reduce the problems in this technique such as errors in manual operation, labor-intensive, time consuming, many undetermined results, difficult to be quantified, batch by batch, and low sensitivity and specificity.
Chemiluminescence Immunoassay
Chemiluminescence immunoassay has been rapidly developed, which has been widely used in domestic and foreign markets, because of the good sensitivity and specificity, wide detection range, stable reagents, and no radioactive contamination. It is mainly used in microplate-based and magnetic particle-based immuno-chemiluminescence detection systems.
The development of chemiluminescence immunoassay system was mainly focused on the development and improvement of two core technologies: solid phase and luminescence system: (1) The solid phases widely used were plastic particles, super paramagnetic particles, elastic plastic tubes, plastic beads, etc. (2) The chemiluminescence systems were enzymatic chemiluminescence, direct chemiluminescence, and electrochemiluminescence.
Classification of chemiluminescence immunoassay
Membrane-Based Immunoassay
Membrane-based immunoassay was a new in vitro diagnostic technology developed on the basis of enzyme-linked immunosorbent assay, fluorescent immunoassay, colloidal gold immunotechnology, and membrane as solid phase. Common methods used were colloidal gold chromatography, fluorescent chromatography, dot percolation tests, dot enzyme immunoadsorption, and immunoblotting, which were easy to operate, without large equipment and less time-consuming.
