Detect antibodies in the blood which indicate whether or not a person, who has now recovered, had been infected by a specific pathogen in the past. The results of these tests, also called serology tests, are especially important for detecting previous infections in people who had mild or no symptoms.
Antibody serology tests are useful in establishing whether a person has acquired immunity to a virus and is also useful in testing whether or not a vaccine against infectious disease is working. There is currently not enough evidence to say whether or not a person infected with SARS-Cov-2 will be immune and protected from reinfection even if they have developed antibodies to the virus.
For the test, healthcare workers draw a sample of blood from a finger or a vein which is then analysed to detect SARS-CoV-2 antibodies that are formed by the immune system in response to infection with the virus. With COVID-19, there are two types of antibody that doctors will test for do:
- IgM antibodies, which develop early on in an infection, and
- IgG antibodies, which are more likely to show up later. While they help fight the virus, it is not clear how long they remain active, and, therefore, how long immunity lasts. Some studies have suggested they last for up to seven months.
Many commercial companies offer COVID-19 antibody tests, some of which are more reliable than others.
Blood tests are generally measured on:
- the sensitivity of the test is important for avoiding “false negatives”, which is when the blood appears free of antibodies against SARS-CoV-2 despite the previous infection with the virus.
- the specificity of the test is important for avoiding “false positives”, or giving a positive result to a different but similar virus (e.g. a coronavirus such as SARS1). Be careful not to confuse the antibody tests which show past infection with the swab or RT-PCR tests which can tell whether a person is currently infected.
Immune System Tests