- What Is the Test?
- Understanding Test Results
- How It Works
- Effects of the Vaccine
If you have gotten a COVID-19 antibody test, you may be confused if your results are reported as equivocal. An equivocal test result means that the result could not be interpreted as positive or negative.
What is a COVID-19 antibody test?
A COVID-19 antibody test, also called serology test, is a blood test that is done to determine whether you have antibodies to coronavirus (SARS-CoV-2), the virus that causes COVID-19. Antibodies are proteins produced by your body’s immune system to fight a particular virus after you have been infected or vaccinated.
It may take about 2-3 weeks (or longer) for your body to develop enough COVID-19 antibodies after infection or vaccination. Antibodies may be detected in the blood for several months or more after recovery from COVID-19.
If antibodies are detected, this indicates that you were infected with the COVID-19 virus, regardless of whether you were vaccinated or whether the infection was severe, mild or asymptomatic. The study of antibody seropositivity in a population helps government officials understand the extent of infection and the cumulative incidence of infection. So it is also used in epidemiology and for public health research.
Understanding COVID-19 antibody test results
COVID-19 antibody test results may be:
- Positive (antibodies detected)
- Negative (antibodies not detected)
- Equivocal (could not be interpreted as positive or negative)
Positive antibody test results mean that:
- You may have antibodies from an infection of the virus that causes COVID-19.
A positive antibody test result does not mean you are immune to COVID-19. It is unknown whether SARS-CoV-2 antibodies will protect you from re-infection, how long the protection lasts and to what extent.
Negative antibody test results mean that:
- You may not have had COVID-19 previously.
- You could have a current infection or have been recently infected.
- You had COVID-19 in the past but have not yet developed a detectable level of antibodies.
Negative antibody test results do not always indicate with certainty that you do not have or have not had an infection with SARS-CoV-2.Some people who are infected may never develop antibodies.
Equivocal antibody test results mean that the results could not be interpreted as positive or negative. Reasons for this may include:
- There is an immune response but it’s not strong enough to give a positive result.
- You may have had an infection in the past caused by another virus in the coronavirus family.
- Faulty techniques or faulty testing kit.
If you received equivocal test results, you may need to take a repeat test.
How does a COVID-19 antibody test work?
A technician will take your blood sample and test for one or both kinds of antibodies:
- IgM antibodies develop early in an infection and indicate active or recent infection.
- IgG antibodies show up later and can indicate past infection. IgG is the type of antibody found in abundance in blood about 14 days after symptoms start. Because antibodies are abundant in the bloodstream, the antibody test is usually a blood test. Though antibodies are also present in saliva, salivary antibody tests may not work as well as blood tests.
- Antibodies against the spike protein can indicate how successful a vaccine-induced response was.
Are there risks to a COVID-19 antibody test?
The risk lies more in the accuracy or inaccuracy of test results:
- False-positive result. Your test result may be positive, but you may not actually have antibodies to SARS-CoV-2 and may not have had exposure to the virus in the past.
- False-negative result. You may get a negative result yet have SARS-CoV-2. You may not have a detectable level of antibodies, or the test was done too soon after infection and your body has not yet developed antibodies.
Whether your antibody test is positive or negative, you may still be able to get infected with COVID-19 or unknowingly spread the disease. So just because a test shows that you have antibodies in your blood doesn’t mean you don’t need to be vaccinated.
How do COVID-19 vaccines affect antibody tests results?
Even if the vaccine was successful at protecting you from COVID-19, there’s a high chance you may get a negative result from a serology test.
This is because different serology tests detect antibodies to different parts of the virus. For example, some tests detect antibodies to the spike protein produced in response to viral infection or the vaccine. Other tests detect antibodies to the nucleocapsid protein produced in response to viral infection but not to the vaccines.
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World Health Organization. "Immunity Passports" in the Context of COVID-19. https://www.who.int/news-room/commentaries/detail/immunity-passports-in-the-context-of-covid-19
United States Food and Drug Administration. Antibody (Serology) Testing for COVID-19: Information for Patients and Consumers. https://www.fda.gov/medical-devices/coronavirus-covid-19-and-medical-devices/antibody-serology-testing-covid-19-information-patients-and-consumers
McIntosh K. COVID-19: Epidemiology, Virology, and Prevention. UpToDate. https://www.uptodate.com/contents/coronavirus-disease-2019-covid-19-epidemiology-virology-and-prevention