Serological tests

Serological tests detect immunoglobulins, which are antibodies capable of binding to infectious agents and activating the immune system. There are different types of immunoglobulins, however, the most commonly used for diagnostic purposes are known as IgM and IgG. IgM immunoglobulin is the first antibody produced by the body when it comes into contact with an infectious agent. It can generally be detected during the first week of infection, but especially after this initial period. IgG immunoglobulin is a more specialised antibody. It binds directly to the pathogen and is therefore an indicator of later stage infection. Its presence usually indicates resistance to a microorganism.  

Serological tests reflect the immune response, and can therefore be used for diagnostic purposes. The presence of antibodies, positive for either IgM or IgG, can indicate whether the person has been recently infected or is recovering from the infection. On the other hand, they also indicate, in subsequent studies, the immunity status of a population. This is particularly helpful when it comes to vaccination campaigns. For SARS-CoV-2 infection, the presence of IgM indicates recent infection while the presence of IgG indicates infection for at least 10 days already. In rare cases, certain individuals are not able to produce antibodies. It is therefore important that serological tests are properly interpreted by trained personnel.  

First a blood sample is taken, then it is centrifuged to extract the serum. Various techniques can be used to detect the presence of antibodies. The ones most commonly used at present, in the context of the SARS-CoV-2 pandemic, are the following: 

• Enzyme-linked immunosorbent assay (ELISA). These are assays based on antigen-antibody binding reactions. An enzyme is used as a marker to measure the formation of these complexes. The technique must be performed in a microbiology laboratory, as specialised equipment and trained personnel are needed to perform and interpret the assay. The assay uses a blood sample and can take several hours.  
• Lateral flow tests, also known as immunochromatography tests. These detect the presence of antigen-antibody complexes in a liquid sample without the need for specialised equipment. The basic principle underlying the technique is very similar to that used in pregnancy tests. The test is dipped into the sample and if the antibody to be detected is present, a coloured line appears indicating a positive result. If there is no reaction, a different coloured band appears. The sample must be blood, serum or capillary blood. This test does not have to be done in a laboratory. Depending on the manufacturer, the result is available in 15-20 minutes.  

You do not need to prepare before the test.