Sars-Cov-2 Rapid Antibody Tests: Interest and Limits

Characteristics of Rapid Antibody Tests

The main peculiarity of a rapid serological test is the speed of its execution and the ease of its interpretation, Opinion which could allow rapid mass screening to initiate an effective health control policy [7]. So, the manufacture of commercial rapid test kits was immediately started as soon as the molecular structure of SARS-Cov-2 was characterized. The principal of most of those commercial kits is based on the colloidal gold immunochromatography assay in order to detect antibodies directed against SARS-CoV2 [8, 9]. But, during covid 19, the antibodies detected by the different methods can be directed against the Nucleocapsid (N), the whole Spike (S) antigen, the S2 subunit of the Spike protein which is common to all coronaviruses or the S1 subunit which is specific to each strain of coronavirus [10].

Several type of kits are available and depending on the isotype of the antibody detected, the companies offer a screening for combined IgM/IgG, or a detection of IgM and IgG separately, while some others offer a screening for IgM alone. The essay is realized according to the producers’ recommendations, either on whole blood which allows their use even at home, or on serum or plasma. However, it is imperative to respect proper specimen handling and storage to maintain the performance of serologic tests.

Example of Some Rapid Antibody Tests

In this short review, we took the example of 4 kits produced by four different companies; the principle of the 4

kits is based on the colloidal gold immunochromatography technology with different variations of the technique. In the Well’s reaction, the test sample interacts with the “Detection buffer” containing different components from a producer to another one, nitrocellulose strips are also coated differently from a producer to another one too. For the first kit, anti- chicken IgY polyclonal antibody and recombinant SARS- Cov-2 antigen, marked the both of them with colloid gold are coated in reagent conjugate pad. Anti-human IgG monoclonal antibody and anti-human IgM monoclonal antibody are coated in test zone of nitrocellulose membrane to bind with the immune complex (specific SARS-Cov-2 antibody contained in the sample/colloid gold marked recombinant SARS-Cov-2 antigen) and form a line in test area (Test line). Chicken IgY is coated in the control area of the strip and bind with the excess of colloid gold marked anti-chicken IgY polyclonal antibody (Control line). For the second one, it uses double antigen sandwich method. The detection line was coated with SARS-Cov-2 recombinant antigen, and the control line was coated with goat polyclonal IgG. During the assay, the antibody anti SARS-Cov-2 in the sample binds first to the colloid gold labeled Cov recombinant antigen to form a solid phase Cov antigen-Cov antibody-labeled Cov recombinant antigen-colloidal gold complex, at the control line position form a solid phase goat polyclonal IgG-labelled Cov recombinant antigen-colloidal gold complex; after completing the test, we observe the Colloidal gold color reaction of Test line and Control line which determine the result. For the third kit, detection of IgM and IgG is separated. During the test, the sample and the detection buffer are applied in the Well reaction, the IgG or IgM anti SARS-Cov-2 here in the sample combined with Colloidal gold-labelled SARS-Cov-2 recombinant antigen forming a complex. During the flow, the complexes move among the strip. When passing the detection line of IgM or IgG anti sars-Cov-2 , the complexes are captured by anti-human IgM (Test line IgM) or anti-human IgG (Test line IgG) resulting colouring on lines; when passed the control line, colloidal gold-labelled DNP is captured by quality-control antibody resulting colouring on the line too. Regarding the last one, only screening for IgM is possible. In fact, when the sample contains the SARS-Cov-2 IgM antibody, it forms a complex with colloidal gold-labelled SARS-Cov-2 recombinant antigen. After the migration, the complexes combine with the coated Mouse anti-human IgM monoclonal antibodies at the test area and develop a colour. Goat anti-rabbit antibodies are coated in the control line to capture the gold labelled quality control antibody (Rabbit IgG antibody) and form a complex viewable with a gold coloration.

Apart the minor differences in the composition of kits and the principle of the assays mentioned above, some other elements vary from a manufacturer to another one, such as: the biological nature of the sample to be tested, the sensitivity and specificity (Table 1). Concerning the first point and according to the recommendations of each producer, the tests can be done on plasma, serum or whole blood, however, the second company does not recommend the use of whole blood, resulting on limit uses of their kits outside laboratories.

Limitations of Rapid Antibody Tests

Although, the rapid surgical assays have multiple advantages, the WHO does not recommend the use of “POCT” in the diagnosis of covid 19 and considers that a larger number of studies on more significant cohorts is necessary to pose this indication [5]. These apprehensions can be explained by the limits of these tests. Indeed, the positivity of the serological tests depends on the seroconversion because the IgM can be detected at the earliest, 3 days after the start of the infection and reach their maximum 2 to 3 weeks later, while the detection of the IgG remains variable; it can be synchronous with that of IgM or occurs later with a peak around the 17th day from the infection’s starting date of [11, 12, 13]. Note that, the time of onset of seroconversion pends on several factors: age, sex, nutritional status, disease’s severity, medication taking and immune status [5]. Furthermore, a lower level of antibodies not reaching the detection threshold of the used test is another point to be taken into consideration. As a result, a negative test does not completely exclude the infection by SARS-COV-2 [5]. In the other hand, we could wrongly conclude that a positive test indicates the patient was previously infected with SARS-CoV-2 because of possible cross reactions. In order to overcome this bias, some laboratories use recombinant SARS-CoV-2 S1 subunit antigen which shows few cross-reaction with others Coronavirus [14]. Regarding the 04 kits cited in this review, no details were provided by the manufacturers about the nature of the antigen used for their assays and finally, rapid serological tests are qualitative and cannot quantify the level of antibodies.

In addition, at this moment, it is well established the evaluation of the sensitivity and specificity of the different serogicals assays must be similar and meet standardized and rigorous recommendations. This is unfortunately not the case for the rapid serological tests used for the detection of anti SARS-Cov-2 Antibodies available nowadays, on the market.