We know that most people who have been in contact with the Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are supposed to develop specific antibodies. If sensitive enough, the serological tests could therefore be a solution during the epidemic, to identify potential healthy carriers, people who have encountered the infectious agent and perform the immuno-monitoring after vaccination.
SEROLOGICAL TEST OVERVIEW
Serological tests allow to detect soluble proteins, hormones or biomarkers – antibodies in the case that interests us today – in a blood serum sample. These tests can be qualitative or quantitative and are widely used for screening, diagnosis, or patient follow-up, but also for epidemiological studies.
As all diagnostics tests these, tools are not 100% reliable. Indeed, due to the time of appearance of immunoglobulins and also their expression in a wide range of quantities – starting from very low levels – the tests must be very sensitive to avoid delivering false negatives (when the biomarker is present, but the test fails to detect it). The question of the limit of detection (LOD) will be discussed below.
On the other hand, the specificity has to be good enough for the test to be meaningful; you do not want to have too many false alerts that would not come from real infections (false positives), although in the case of Covid-19, the risk for the patient is relatively low since the consequence will just be a quarantine, as far as no specific treatment can be given (present situation). A true positive result means that the patient has been in contact with a pathogen at some time. Unlike virological tests in which viral particles are detected, serological tests allow a patient to be evaluated even if the virus is no longer present. In addition, long-term follow-up can provide information, for example if the patient is in the course of its isotype switch.
There are different types of serological tests:
- Precipitation tests based on the precipitation that takes place when antibodies (especially IgMs) and antigens are mixed together.
- Neutralization tests, which depend on the capacity of antibodies to neutralize the infectious properties of the infectious organisms.
- Hemagglutinin-inhibition tests based on the agglutination of red blood cells. This agglutination will be prevented by the presence of antibodies.
- Enzyme Linked Immunosorbent Assay (ELISA) or Lateral Flow Immunoassay (LFIA) based on antibody binding to the antigen on a solid surface (plate or strip)
On this article and White Paper, we will focus only on these last two techniques which are the most common in routine.
