As COVID-19 has spread around the world, tests to track and control outbreaks are increasing. In addition, to an increase in RT-PCR testing for acute infections, there is a need for serological testing to assess who may have acquired immunity to future SARS-CoV-2 infections. These tests identify people with neutralizing antibodies against SARS-CoV-2 by using coronavirus proteins as bait.
1) Main types of antibody tests for COVID19
Hundreds of companies have developed serological tests of varying quality. There are many types of tests, but how do they work and what are the limitations of each?
2) Detection of COVID-19 by ELISA test
ELISA (enzyme-linked immunosorbent assays) are the simplest technologies for quantifying an analyte in a sample. Typically, 96-well plates are used that are coated with a capture agent. In most cases, these capture agents are polyclonal antibodies to the analyte. For COVID-19 serological tests, these are whole recombinant viral proteins or fragments.
As the samples incubate, the capture agent binds and isolates the analyte from the sample. After washing to remove unwanted molecules, a detection agent is added. The detection agent is typically a monoclonal antibody against the analyte conjugated to an enzyme such as horseradish peroxidase (HRP). For COVID-19 serological tests, these can be another recombinant protein or an anti-immunoglobulin antibody conjugated to an enzyme. The addition of the enzyme substrate produces a colorimetric signal proportional to the amount of substrate, allowing quantification.
The main advantage of this method is that there are well established protocols for development and quality control. It is also a very common type of test used to report medical diagnoses, so it is compatible with most clinical laboratories. They also require active participation from the researcher and a significant investment of time.
3) Chemiluminescence-based COVID-19 tests
CMIAs (chemiluminescent microparticle immunoassays) are another method developed for the detection of serological tests against COVID-19. In this assay, the capture agent is coupled to a paramagnetic microparticle rather than coating a plate. After washing, the detector agent is added with a chemiluminescent conjugate. After formation of the immune complex and washing, the other component of the chemiluminescent conjugate is added and the resulting signal is read and used for quantification.
Unfortunately, as sensitivity increases, the possibility of false positives increases and this can be a problem with this method in certain circumstances.
4) Antibody test against COVID-19 based on lateral flow
The lateral flow test is a chromatographic test. In this test, samples flow through a strip due to capillarity and provides a simple yes / no reading. Samples first pass through a conjugate pad containing the bait conjugated to a fluorophore, gold, or other tag. Subsequently, the immune complexes travel past a line of capture antibodies that immobilize them. The concentration of the immune complex in a line produces a readable signal.
The remainder of the sample is then passed through a final antibody line to produce an internal positive control. The main advantage of this test is simplicity. Depending on the reading, if the health authorities consider it appropriate, they could be used at home or at control points enabled for patient tracking. Also, lateral flow tests are generally safe at room temperature.
This simplicity is a double-edged sword. At this time, the level of binding necessary to reasonably conclude the presence of neutralizing antibodies is unknown. This makes the test prone to false positives or negatives.
5) Recommended Genetex Antibody Pairs for Lateral Flow
|Pair I||Nucleocapsid antibody [HL5511]||Rb recAb||14 pM||GTX635689|
|Nucleocapsid antibody [HL448]||Rb recAb||11 pM||GTX635686|
|Pair II||Nucleocapsid antibody [HL5410]||Rb recAb||9 pM||GTX635685|
|Nucleocapsid antibody [HL455]||Rb recAb||6 pM||GTX635688|