Who Should Get an Antibody Test for COVID-19? Here's What Doctors Advise
More than a year and a half into the COVID-19 pandemic with three vaccines available in the US, it seems like everyone knows someone—or has at least heard of someone—who's been vaccinated and diagnosed with the virus thereafter. So it should come as no surprise that many people are wondering whether their shot is still doing its job.
Enter, SARS-CoV-2 antibody blood tests, which detect the infection-fighting proteins that linger after your immune system beats COVID-19, or revs up in response to a vaccination. But can these tests actually gauge whether you have COVID-19 immunity?
Turns out antibody tests are definitively not recommended to determine whether you're protected from COVID-19, according to the US Food and Drug Administration (FDA). And experts don't see great value in these commercial tests prescribed for other reasons (i.e., to find out whether you've had COVID-19). Here's what you need to know about antibody tests for COVID-19, and who (if anyone) should get an antibody test.
What is an antibody test, and what is it typically used for?
Also known as serology tests, antibody tests, which require a doctor's prescription, are blood tests that detect the presence of SARS-CoV-2 antibodies in the bloodstream. Antibodies are protein molecules produced by the immune system when your body fights off a pathogen or undergoes vaccination; they help defend your body from said pathogen the next time you're exposed to it, and can also help reduce the severity of symptoms in the case of reinfection.
Antibody tests are typically used to screen for evidence of past infection or preparedness to fight off a specific virus—i.e., evidence of effective vaccination—with varying degrees of accuracy, according to the Centers for Disease Control and Prevention (CDC). While antibodies can protect people from getting an infection or super sick from that infection, protection differs by individual and disease, also according to the CDC.
Commercially available tests screen for two different kinds of antibodies and can tell you how many you have, according to Dr. William Schaffner, MD, professor of infectious disease at the Vanderbilt University Medical Center in Nashville:
- Spike protein antibodies Imagine a COVID-19 virus is a tennis ball with key-like spikes, and the cells they infect are locks. Typically spikes, or spike proteins, lock up with your cells to get inside and start multiplying. Vaccines cause the body produce antibodies that glob onto the spike proteins like bubble gum so the keys can't get in the locks.
- Nucleocapside antibodies These are produced in response to infection. Nucleocapside refers to the tennis-ball part of the virus; neucleocapside antibodies prevent this area of the virus from getting into your cells.
Should you get an antibody test to see if you've had COVID-19?
In theory, testing positive for spike protein antibodies should mean your vaccine is working, and a positive nucleocapside antibody test should confirm you've had COVID-19. But—and this is big but—the jury is out on how many antibodies you need for immunity, and how long that immunity might last, according to the FDA. What's more: "We don't know whether antibodies protect against various COVID-19 strains in circulation," Dr. Cheryl G. Healton, DrPH, dean of the School of Global Public Health at New York University, tells Health. "So there's not much value in antibody testing."
Dr. Schaffner—and the majority of his infectious disease colleagues—agree: "In medical school, we're taught you should never do a test unless you know what you're going to do with a positive or negative result," he tells Health. "In the case of antibody testing, the results can't be interpreted, which is why we don't recommend it at all."
If there's one thing literally every expert and governing body agrees on, it's that antibody presence should not take the place of vaccination or boosters among those who are eligible.
Should you get an antibody test to see if you presently have COVID-19?
No. Because it can take one to three weeks after infection to detect virus antibodies, these tests aren't your best bet for diagnosing COVID-19 on the spot, according to the CDC.
That's not the only reason why you shouldn't rely on an antibody test to tell whether you have COVID-19: The blood tests are prone to false negatives among those who don't have enough antibodies yet, and false positives among those who've been exposed to coronaviruses besides COVID-19, according to the FDA.
The good news: There are tests can accurately check for current COVID-19 infections, per the CDC—those are known as viral tests which examine the sputum from your nose or mouth for signs of the virus. The CDC lists two types of viral tests commonly used to test for COVID-19: nucleic acid amplification tests, or NAATs (you may know this type of test by one of its main types, the reverse transcription polymerase chain reaction, or RT-PCR test), and antigen tests. NAATs identify the genetic material of the virus in samples, while antigen tests identify the presence of antigens.
Should you get an antibody test to see if your COVID-19 vaccine is still working?
"No," says Dr. Healton—and you'd be hard-pressed to find an expert who disagrees.
Antibody testing is not currently recommended to assess for immunity to COVID-19 following vaccination, according to the CDC, which notes that these diagnostic tools have variable sensitivity, specificity, and positive and negative predictive values. In other words? They just don't tell you that much, and aren't especially accurate in assessing protection from COVID-19.
Even in the case of a positive result, the jury is out on just how many antibodies result in production. In other words, you might have some degree of protection, but don't know whether you're holding a plastic or ironclad shield, and whether your body is prepared actually prepared to fight COVID-19.
It should go without saying that you shouldn't get an antibody test to assess your need to get the second shot in your vaccine series, or a booster—just get them.
Are COVID-19 antibody tests ever helpful?
"In clinical research settings where studies use highly validated research protocols for utmost accuracy, antibody tests can be useful," offers Dr. Schaffner, who can't think of a great use case for a commercially available antibody test.
Because these tests won't definitively tell you if you have, had, could get, or could spread COVID-19, and because a positive test could give you a false sense of security, experts generally recommend against COVID-19 tests in all circumstances outside of research laboratories.
The bottom line here: Antibody testing is not recommended to check and see if your COVID-19 vaccine is still "working," and experts agree there's no huge value in testing to see if you've previously been infected with the virus. However, "if your health care provider wants to know whether you've been infected with COVID-19, it may be helpful," says Dr. Healton. But take the results with a grain of salt and without plans to act on them: "Being antibody positive is not a substitute for vaccination."
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