COVID-19 Antibody Tests

COVID-19 serological tests identify early- and late-stage antibodies against this virus that are present in the blood. These tests show two types of antibodies: the early type called IgM and the late-type, called IgG.

This test is best used to discover if someone is going through, or has gone through the infection and has already recovered. It is less reliable in detecting an ongoing infection in asymptomatic, or mild-onset, than the PCR test.

An antibody serological test is recommended in the following cases:

  • To test contacts of people infected with COVID-19.
  • For detection of ‘early-stage’ IgM antibodies against COVID-19. Used to check the immune response in the infected
  • In asymptomatic individuals, an antibody test can be used to show that they have developed protection without being sick. The doctor may recommend PCR testing to rule out current infection.

Antibody production

  • Immunoglobulin M (IgM) – IgM antibodies are produced as a body’s first response to new infections, or to a new “non-self” antigen, providing short-term protection. They increase for several weeks and then decline as IgG production begins.
  • Immunoglobulin G (IgG) – About 70-80% of the immunoglobulins in the blood are IgG. Specific IgG antibodies are produced during an initial infection or other antigen exposure, rising a few weeks after it begins, then decreasing and stabilizing. The body retains IgG antibodies that can be rapidly reproduced whenever exposed to the same antigen. IgG antibodies form the basis of long-term protection to prevent re-infection.
  • Immunoglobulin A (IgA) – IgA comprises about 15% of the total immunoglobulins in the blood but is also found in saliva, tears, respiratory and gastric secretions, and breast milk. IgA provides protection against infection in mucosal areas of the body such as the respiratory tract (sinus and lungs) and the gastrointestinal tract.

Negative test results

A negative test result means that the antibodies to COVID-19 were not detected in your sample. However, it is possible for this test to give a negative result that is incorrect (false negative) in some people.

A negative result may occur if you are tested early in your illness and your body hasn’t had time to produce antibodies to the infection. This means that you could possibly still have COVID-19 even though the test is negative.