Immunoassay refers to the chemical test administered to quantify or detect specific substances with the use of an immunological reaction. Since purified antigens and antibodies are used in this process, you can say that immunoassay is specific and highly sensitive.
Immunoassays can be quantitative or qualitative, too. Quantitative immunoassay can be utilized to determine analyte concentration. This is done by measuring the strength of the signals produced when the indicator reacts. Qualitative immunoassay, on the other hand, can be used in situations such as identifying whether or not a woman is pregnant. In this case, pregnancy is detected as early as two days after fertilization.
The principle behind immunoassay
In an immunoassay test, an antibody that specifically binds to an antigen of interest is being used. For an antibody to qualify for immunoassay use, it must exhibit a higher antigen affinity. In normal immunoassay tests, these antibodies are either polyclonal or monoclonal.
Polyclonal antibodies may be inexpensive, but they can recognize different antigen-binding epitopes. However, these antibodies are less specific. Monoclonal antibodies, on the other hand, are so precise that they only bind to a very specific antigen. As such, they also tend to yield more specific and accurate results. The only downside is, unlike the polyclonal ones, they are more costly.
ELISA stands for Enzyme-linked Immunosorbent Assays, and this is a test that aims to detect antigen of interest concentrations. Currently, this test is widely used to examine blood samples for viruses like HIV.
Aside from ELISA, another type of immunoassay test is called enzyme immunoassay or EIA. Here, the antibodies used to check the presence of an antigen of interest are labeled with the specific indicator enzyme responsible for enabling minute quantities of antigens of interest to be traced.
Next to ELISA and EIA is RIA, short for radioimmunoassay. In this kind of text, a radioisotope is used to trace the presence of the antigens of interest. Currently, RIA is used to screen blood samples of people who are suspected of Hepatitis B infection. In RIA, the Hepatitis B antigen is being detected with the use of a radioisotope. The only problem with RIA is that it raises serious issues like safety and labor-intensiveness.
Why we need immunoassays for COVID-19
Currently, we use Reverse Transcription- Polymerase Chain Reaction (RT-PCR) to check whether or not a person is COVID-19 positive. Since SARS-CoV-2 is an RNA virus, the PCR test is preceded with a reverse transcriptase enzyme test. While the PCR test magnifies the sample DNAs for it to be analyzed and tested, the reverse transcription test, on the other hand, works by producing a complementary DNA template from the RNA virus.
RT-PCR may seem to be a successful testing procedure because of its sensitivity and speed, but it has its limitations. Since the SARS-CoV-2 loads in a person’s respiratory tract vary considerably, there are higher incidences of false-negative results. Since probable cases continue to remain negative after a number of swabs, the repetitive process further exposes healthcare workers to the infection.
This is where immunoassays provide the most value. Immunoassay can detect past infections. Even if the person already recovered from the disease and his body has been cleared of the virus, an immunoassay can still trace the infection retrospectively. Through this, you can identify the stages of infection and draw estimates as to when the person got exposed. The data you can draw here can significantly aid in contact tracing.
Then again, since the health community is learning more about the virus on the fly, one cannot say that immunoassay is the definite solution to address the accuracy of COVID-19 testing kits. Though more study is required to institutionalize this method, for now, what’s clear is the fact that immunoassay can significantly help in the government’s effort of tracing the route of infection.