Covid-19, which is also referred to as Coronavirus diseases 2019, which is caused by the SARS-COV-2 virus, has affected millions over the past two years, and the numbers continue to increase every day. SARS-COV-2 is a part of the family of Coronaviridae, which is a type of RNA virus. Four types of coronavirus are found to exist so far, which include: Alpha, beta, gamma, and delta coronavirus. A majority of the coronaviruses which infect humans are associated with respiratory symptoms, which range from mild to moderate intensity (1)
Several types of Covid-19 diagnostic tests have been introduced by different companies all over the world, with different mechanisms of action and delivery times. Each category of diagnostic tests may further be divided into several subtypes, which may be appropriate for unique case types. The most widely used type of covid-19 diagnostic test is the Molecular test, which detects the viral particles in the blood. These tests do not only allow the detection of the disease but also control its spread and further mutation. One such test is the Polymerase chain reaction, which allows the detection of the viral RNA in the samples taken from the patient’s throat and nasal swabs. The results of such tests tell the test taker whether they are currently infected with the virus.
Another commonly conducted type of Covid-19 test is the Antibody test, which helps identify the presence of three different antibodies in the body which may have developed due to an immune response to foreign particles or virus in the body. These antibodies include the IgA, IgM, and IgG antibodies.
IgG, which stands for Immunoglobin G, is the most frequently found protein in the human blood and is equal to at least 10 to 20% of the entire protein count of the plasma. IgG can be divided into numerous subtypes, which are names IgG1, IgG2, IgG3, and IgG4.
These subtypes were initially discovered in the 1960s after detailed studies were conducted on human IgG myeloma proteins and rabbit antisera. The subtype of IgG, which is found to be most abundant in the blood, is IgG1, which is followed by IgG2, IgG3, and the least abundant being IgG4. (2). Each of these subclasses has its unique properties in the body, meanwhile being almost exactly identical in structure and composition.
The IgM antibody is another abundant form of antibody found in the body, and the isotopes of which can be seen in hexameric and pentameric forms. Each monomer of the IgM antibody is approximate 190 kDa and is composed of a combination of heavy and light chains. While the heavy chain has five domains known as Vµ, Cµ1, Cµ2, Cµ3, and Cµ4, the light chains have only two domains known as Vκ-Cκ and Vλ-Cλ. (3)
A large number of the IgM antibodies secreted in the body are the ‘natural IgM antibodies’ and are found in the peritoneal and pleural compartments of the body. (4) Studies have found this class of antibodies to be present in all types of jawed vertebrates as they are evolutionarily conserved. (5) These natural IgMs are found to be encoded by unmutated germline gene segments. Moreover, as compared to the other antibodies, the IgM antibodies are also found to exist in higher amounts in the neonates of both humans and neonates. (6)
At least 66 mg/kg of IgA are produced in the human body each day. This makes IgA the second most common antibody found in the blood and the most commonly found antibody at mucosal sites. What makes this antibody unique is that it is non-inflammatory in nature and helps maintain the homeostasis in the mucosa. (7)
Even though it does not directly impact the inflammatory activity of the body, it has the potential to either inhibit or initiate inflammation in the body via interactions of different receptors. These interactions can either be active or passive in nature.
Unlike other diagnostic tests, the antibody tests do not have the ability to let the patient know if they are currently infected with the covid-19 infection; however, studies have shown some significant benefits of these over the other tests. Some commonly discussed benefits of antibody tests include:
What makes these tests an excellent alternative to the more commonly used antigen tests in their ability to allow surveillance of a large population group, prove as a reliable alternative test for those who have been diagnosed negative for the RNA test, may allow the detection of previously diagnosed or undiagnosed virus infection and efficient monitoring of the body’s immune response. (8) It also acts as a helpful tool for identifying potential plasma donors who can help the critically ill and scientific studies with their antibody-containing plasma.
Two types of antibodies may be detected by this test, which includes the Neutralizing and Binding Antibodies. While the Neutralizing antibodies are found to bind with viral cells in order to decrease their infection rate, the Binding antibodies are only able to bind with the viral particles without affecting their infection rate.
The neutralization antibody tests are considered one of the standard testing methods for covid-19 serology of the blood serum and may also act as ways to diagnose the performance of binding antibody tests. They use the interaction of the virus and the antibody as an advantage and are usually conducted in a test tube or micro titter plate. Firstly a diluted serum, also known as the antibody solution, is prepared, which is then incubated with a specific quantity of the virus. This results in the formation of immune complexes, which can then be added to the cell monolayer. After being incubated for numerous days, the plaques get formed, which may be seen using fluorescent antibodies or special dyes. PRNT is usually considered the gold standard for neutralizing antibody detection; however, the work and time that it requires do not allow it to be used on a larger scale.
The Micro neutralization antibody test is another type of test which allows the detection of viral antigens in cells of the body which have been infected with the virus. This is usually conducted in microtiter plates and in combination with methods like ELISA, which can show results in as few as two days. The incubation period for this is usually overnight, after which the viral antigen is detected in the microtiter plate using ELISA. If the viral antigen is detected in this test, it indicates the absence of the neutralizing antibodies at that particular dilution of serum.
The Binding Antibody tests are more frequently conducted as compared to the neutralization antibody tests and are usually in the form of sandwich immunoassays. Various types of binding antibody tests have been introduced for the detection of antibodies like IgA, IgM, and IgG, which include ELISA, LFIA, CLIA, Fluorescence immunoassay, protein microarrays, immune fluorescence assay, and biosensors. (9)
A popular example of the Binding Antibody Test is the WANTAI SARS-CoV-2 Ab Rapid Test which was developed by Beijing Wantai Biological Pharmacy Enterprise Co., Ltd. They are known to conduct the chromatographic lateral flow sensor test in the form of a cassette. As an example of lateral flow sensing, the WANTAI SARS-CoV-2 Ab Rapid Test (Beijing Wantai Biological Pharmacy Enterprise Co., Ltd.) employs a chromatographic lateral flow sensor device in a cassette format.
Antibody tests can sometimes be associated with false-negative results due to a lack of sensitivity of the test or an inadequate amount of antibody in the specimen. A lack of sensitivity of the antibody test may be caused by the format of the assay being conducted, the type of antigens being targeted, the quality of the antibody being tested, and variability of the isotypes of the antibodies to be detected. On the other hand, an inadequate quantity of antibodies in the specimen may be influenced by the time of sampling and the diagnostic testing window.
Antibody tests may also be affected by false-positive results, where the tests may indicate the presence of antibodies for Covid-19 in the patient’s blood even though they were never infected with it. This type of error may be caused by the cross-reactivity of antibodies or the contamination or mix-up of samples during testing, packaging, or transport.
Spectrum Bias is another type of antibody error that may occur due to variability in the test performance between different subgroups of the population. (10) This difference may be due to the different genetics of these population groups, which may lead to different sensitivity, specificity, and likelihood ratios. Hence it is up to the clinicians, researchers, and poly makers to consider how the results may vary when the tests are conducted on different population groups.
The ideal candidate for a Covid-19 antibody test is someone who:
The antibody test is an excellent indicator of the late stages or the recovery stage of the Covid-19 virus and may even allow the test taker to know if they had been previously infected. This type of testing is not only beneficial to the test taker but also has contributed greatly to scientific research and the development of effective vaccines for the virus. However, one must keep in mind that higher levels of diagnostic testing are not the key to preventing the spread of the Covid-19 virus, but the immediate isolation, contact tracing, and management of the symptoms have equal contribution and require attention as well.