SARS-CoV-2 is an enveloped positive-sense single-stranded RNA virus. It primarily spreads between people via aerosols and respiratory droplets. The virus infection can result in severe respiratory syndromes (fever, cough) or pass without symptoms.
According to the WHO recommendations, SARS-CoV-2 is identified with tests based on the RT-PCR technique that discover if the genetic material of the new coronavirus is in a tested sample. In addition to this technique, serological tests define specific antibodies to SARS-CoV-2. The serological tests are vital at the last stages of the infection because they prolong the period when the virus can be detected.
The human blood and saliva may contain several types of antibodies that have different structures, concentrations, and biological parameters. For example, during a laboratory SARS-CoV-2 virus examination, IgG, IgM, and IgA antibodies are used.
Tests for the serological diagnostic
You may buy tests based on a solid-phase type of enzyme immunoassay (EIA), for example, Anti-SARS-CoV-2 ELISA tests, as well as cassette devices based on the lateral flow immunochromatographic assay or so-called rapid tests.
1. Elisa Test
- Designed for medical labs, they have been used for years as a proven diagnostic tool to identify antibodies during multiple infectious diseases. Every SARS-CoV-2 ELISA test kit contains a pre-coated microplate with separate reaction vials covered by the recombinant Spike protein.
- At the first stage of analysis, the antigen is incubated with patients’ diluted serum or plasma samples. If the tested material contains specific antibodies, there is an antigen-antibody complex.
- The second stage of incubation is about detecting bound antibodies marked with a conjugated human enzyme. Finally, an added substrate for this enzyme changes color upon reaction. The higher the concentration of the primary antibody present in the serum, the stronger the color change.
2. Cassette Test
This is a simplified enzyme immunoassay (EIA) used for primary discovering SARS-CoV-2 antibodies. A sample material (capillary blood, venous blood, serum, or plasma) is attached to a chromatographic strip and then migrates through the nitrocellulose membrane by capillary action. If there are antibodies to the new coronavirus, they will be bound.
At this stage arises a complex with an antigen marked with colloid gold. Then, during the chromatography, it is shifted to the reaction zone where the complex meets human antibodies immobilized on the membrane surface. The more complicated complex is trapped, forming a burgundy-colored band that confirms a reactive test result.
Difference between EIA and cassette tests
Identifying antibodies using the EIA technique requires incubation. Then, the results are determined spectrophotometrically at a specific wavelength. In the case of ELISA tests, the virus concentration is calculated based on the test sample consuming ratio, assuring a quantitative and qualitative result. That method guarantees a high level of study reliability.
The cassette tests can be used both in medical labs and at home. The reaction results are determined visually by assessing the test band color grade. However, those results are only qualitative and have possible human errors. The quantity of SARS-CoV-2 antibodies cannot be assessed with a cassette test because the test band color grade is not always connected with the concentration of specific antibodies in the tested sample.