How accurate are antibody tests?

Antibody tests, also called serology tests, are used to detect the presence of antibodies to SARS-CoV-2 (the virus that causes COVID-19) in your body, which can indicate a past infection. Antibody tests will not tell you if you currently have COVID-19. 

What are antibodies?

Antibodies are proteins in your blood and mucus that are activated by the immune system to fight against foreign substances, like an infection or a vaccination against infection. There are multiple types of antibodies that emerge as a response to SARS-CoV-2, called immunoglobulins A, G, and M (IgA, IgG, and IgM, for short). Generally most antibody tests look at IgM and IgG. 

Antibody tests are very accurate at detecting whether antibodies are present or not. 

As of October 2021, there are nearly 100 different antibody tests that are authorized by the US Food and Drug Administration (FDA). Most of these antibody tests have >95% sensitivity, meaning that about 95% of people with antibodies will correctly receive a positive result. 

But there are two factors that can reduce the accuracy of antibody tests.

1. Getting an antibody test too soon after having COVID-19 may give you a false negative, as it takes 3 weeks after infection to produce antibody levels that are detectable. 
2. Antibody testing in an area with low prevalence of COVID-19 in the population (meaning that a low percent of total people that have been infected) increases the likelihood of false positives. 

A positive/reactive antibody test result does not mean you are protected against infection.

Positive antibody test results tell you that antibodies are present in your blood. But it does not tell you if you still have natural immunity defending you from re-infection. This is because we still do not know the “correlate of protection” for SARS-CoV-2, or the level of neutralizing (aka virus-blocking) antibodies needed for immunity, and how long the specific antibodies will remain in your body. 

Antibody tests also cannot tell you if your vaccine is still working. 

Currently the US FDA does not recommend using antibody tests to determine if your vaccine is effective. Once scientists figure out the “correlate of protection”, antibody tests could be used in the future to verify whether your vaccine is still producing sufficient neutralizing antibodies for protection or if you need a booster shot. 

So, what are antibody tests used for right now?  

Right now, antibody testing is most useful for epidemiological studies and for vaccine and treatment development. For example, scientists might conduct lots of antibody tests to figure out what percent of a population had COVID-19 at some point. Or doctors may give you an antibody test to confirm a previous infection if you want to donate convalescent plasma, which can be given to hospitalized people with severe COVID-19 as a treatment. 

Last update: 30 September 2021, 12pm ET

Science review: HAY, MG