A: Antibody tests, or serology tests, test your blood for evidence of a past encounter with the SARS-CoV-2 virus.
Importantly, these tests don’t tell you when you were infected, with which variant, or how well protected you are. Antibody tests are not cheap and are usually done in a lab.
For most of us, the main value of antibody tests is to satisfy your curiosity and help solve your COVID mysteries, like those suspicious symptoms that never tested positive.
Note: Antibody tests are NOT the same thing as rapid antigen tests, which detect active COVID-19 infections.
The story of a dear friend is a great example of antibody tests in action. Her husband and three kids all had COVID-19 over the winter break, but she had no symptoms and never tested positive despite several antigen tests. A month later, it was bothering her not to know whether she too had hybrid immunity, so she shelled out for an antibody test. The test came back positive. It seems that her body mounted such a stellar immune response that she never had symptoms, nor enough virus to turn an antigen test positive (she never did a PCR test). Since antibody tests can be positive for over a year after infection, we can’t say for sure that she was infected at the same time as her family.
ANTIBODY TESTS 101
COVID-19 antibody tests look in your blood (a tiny amount) for antibodies against a piece of the SARS-CoV-2 virus. These antibodies are a clue that your immune system has “seen” and responded to either a vaccine or a viral infection. You can think of antibodies as dogs trained to sniff out a very specific scent.
Antibody tests come in several different “flavors”. Some look for antibodies to the viral S (spike) protein, some look for antibodies against the viral N (nucleocapsid) protein, and others look for both or either. Antibody tests also vary in the “breed” of antibody they detect – either IgM (first responders), IgG, or both. Lastly, some antibody tests detect powerful “neutralizing” antibodies, which stop the virus from entering your cells, while others detect plain vanilla “binding” antibodies.
Antibody / serology testing can be done through a lab or a rapid home test. In our Nerdy opinion, lab tests are your best bet. Home rapid tests can be problematic for a few reasons: they are intended for professional use, they are usually sold in boxes of 20 or more, and they often target both the N and S protein, which complicates interpretation (more below).
Antibody tests are widely used in public health research to track how many people have been infected (seroprevalence studies).
WHAT ANTIBODY TESTS TELL YOU
A positive or “reactive” antibody test means that antibodies against a specific piece of the SARS-CoV-2 virus were detected. Some tests give a more detailed score reflecting antibody levels.
If you are vaccinated, you need an N (nucleocapsid) antibody test to know if you have been infected in the past. A positive N antibody test always indicates a prior COVID-19 infection, regardless of vaccination status. By contrast, a S (spike) antibody test is ambiguous if you are vaccinated. Your immune system makes anti-S antibodies in response to vaccines and in response to infection. In the story above, our friend used the N antibody test.
If you are unvaccinated, any type of antibody test (N, S, or both) will tell you whether or not you have been infected in the past.
Most approved antibody tests work very well, with over 95% sensitivity and specificity, but false positive or false negative results are still possible. The chances of a false result depend on many factors including the test brand, the timing of the test, and your personal situation (e.g. pre-test probability of testing positive).
⚠️Unapproved antibody tests are NOT reliable.
WHAT ANTIBODY TESTS DON’T TELL YOU
➡️ When you were infected. A positive test only tells you you were infected sometime in the past few weeks to months. Some people still have detectable antibodies over a year after infection!
➡️ Which COVID variant you were infected with in the past.
➡️Your level of protection from infection and severe disease. Your risk of re-infection and of risk of severe disease depends on many factors, like vaccination status, time since vaccination/infection, immunocompetence, your comorbidities, and the viral variant.
✅ Expect to pay $50-$200 for lab tests
✅ The best time to test is 3 weeks after a suspected infection but up to 4 months is a safe bet. In many cases, antibodies persist much longer than this.
✅Use an approved test through a professional testing lab (see links below).
✅ Use the right type of test for your question (N / nucleocapsid for natural infection).
✅ Recognize that false positives and negatives are both possible (check the test stats).
✅ Know what you will learn. Antibody tests cannot predict your level of protection. Nobody is invincible, even with hybrid immunity.
To take the plunge, contact your doctor and the links below to get started.
Prevalence and duration of detectable SARS-CoV-2 nucleocapsid antibodies in staff and residents of long-term care facilities over the first year of the pandemic (VIVALDI study): prospective cohort study in England