What are COVID-19 antibodies?

Antibodies, or immunoglobulins, are proteins that the immune system produces in response to an infection. Antibodies are divided into classes — IgM and IgG.

Immunoglobulin M (IgM) is produced in the first week of illness and is a marker of the acute stage of the disease. However, the IgM antibody test often gives a false-positive result — for example, if a person is infected with another virus or there is some inflammatory process in his or her body.

In the standard course of infectious disease, the IgM antibodies disappear after a month. They are replaced by IgG antibodies, which are a marker of the disease and "immunity" to it. But with COVID-19, IgM may persist for up to a month and a half, or even more, from the onset of symptoms.

Immunoglobulin G (IgG) starts synthesizing as early as 10 days after the onset of the disease and peaks after 5–6 weeks. It does not make sense to take the test earlier than that.

Why take an antibody test?

As discussed above, the results of the test for IgM antibodies are not always reliable, so it can not be relied on solely to determine the absence or presence of COVID-19. The PCR test is much more reliable and accurate. IgG antibody tests are useful in several cases, including the following:

• You don't know if you have had COVID-19 and want to find out
• You had the disease some time ago, and you want to know if you have developed immunity
• You are planning to get vaccinated against COVID-19

How do you interpret your test?

Antibody tests are qualitative and quantitative. The qualitative aspect states whether you have had the disease. The quantitative aspect determines the titer (intensity) of immunity. The higher the figure, the stronger the person is protected against the virus. So what are the results, and what do they indicate?

IgM (+) detected, IgG (-) not detected:

• Sign of the acute phase of the disease, especially when symptoms are present.
• A possible false-positive result (in chronic diseases).

IgM (+) detected, IgG (+) detected:

• Possible current infection, the onset of convalescence, or recent illness.

IgM (-) not detected, IgG (+) detected:

• You have had COVID-19.

A positive qualitative test result is more than 1.1 IgG antibodies.

A borderline result is between 0.8 and 1.1. Repeat the test in 2 weeks.

A negative result is less than 0.8. The person either has not had COVID-19, or the disease is at an early stage.

13 June 2021