What Is an Antibody Test?
An antibody test is a screening for things called antibodies in your blood. Your body makes these when it fights an infection, like COVID-19. The same thing happens when you get a vaccine, like a flu shot. That’s how you build immunity to a virus.
You may also hear it called a serology test.
The antibody test isn’t checking for the virus itself. Instead, it looks to see whether your immune system -- your body’s defense against illness -- has responded to the infection.
How Does It Work?
You’ll have to give some blood, like through a finger prick. Doctors test for two kinds of antibodies to SARS-CoV-2. They look for:
- IgM antibodies, which develop early in an infection, and
- IgG antibodies, which are more likely to show up later after you’ve recovered
It takes your body about 4 weeks to develop IgM antibodies. But scientists aren’t sure how long it’ll take for this to happen with SARS-CoV-2. More tests are needed to find out.
Keep in mind that current antibody tests can’t tell you if you’re immune to COVID-19. That's because we don't know how long these antibodies might protect you against the coronavirus. And these tests shouldn’t be used to diagnose the virus.
How Does an Antibody Test Work?
A technician will take a bit of your blood, like through a finger prick. The test looks for one or both kinds of antibodies to SARS-CoV-2, the virus that causes COVID-19:
- IgM antibodies, which happen early in an infection
- IgG antibodies, which are more likely to show up later
Most people have IgG antibodies about 7 to 10 days after COVID-19 symptoms start. They usually stay in your blood long after the infection goes away, but we don’t know how long that is for the new coronavirus.
What’s the Difference Between a Coronavirus Test and an Antibody Test?
A coronavirus test, sometimes called a diagnostic test, looks for signs of active virus. It’s simpler and faster than an antibody test. But it tells you only if you have the virus in your body at the moment when you’re tested.
An antibody test shows that you had the virus at some point in the past. It could be gone, or you could still be contagious.
Why Do We Need Antibody Testing?
You could have SARS-CoV-2 and not know it. Not everyone who gets it has symptoms. Experts hope antibody tests can give health officials a better idea of how common the virus is.
Once scientists know who has had the virus, they can find out how sick it makes most people. And they can study what happens if people who've had it come into contact with it again. Along with other scientific information, this can help researchers understand who might be immune to the virus.
The hope is that people with antibodies to COVID-19 can safely get back to work, and normal life, quicker.
These tests may also help with an experimental treatment for COVID-19 called convalescent plasma. Plasma is the liquid part of your blood.
Researchers are studying how antibodies in plasma donated by people who’ve recovered from COVID-19 might help those who are ill with the virus. One theory is that this plasma may help sick people get better faster. But more research is needed.
You can volunteer to donate plasma through the National COVID-19 Convalescent Plasma Project. Or ask your local blood donation center for information.
Are Antibody Tests Accurate?
Companies make their own claims about the accuracy of their antibody tests. Some say it’s up to 100%. Government researchers are studying how well the tests are working, but it’s too early to say for sure.
The FDA says it will crack down on any manufacturer that sells a bad test.
It’s important to note that some tests can mistake IgM antibodies from other coronaviruses, such as common cold strains, for SARS-CoV-2 antibodies.
Who Should Get an Antibody Test?
If you think you might have come into contact with the coronavirus, or if you’ve tested positive for COVID-19 and have fully recovered, you can probably get tested for antibodies.
How Can You Get an Antibody Test?
You can't do these tests at home. But they’re becoming more widely available in many areas. The FDA has issued emergency use rulings for several antibody tests so people can get them before they have full FDA approval. Ask your doctor or local hospital how to get tested.
The National Institutes of Health (NIH) wants to study blood from 10,000 healthy people who haven’t tested positive for COVID-19. Researchers want to see how many people might have had the virus without knowing it. If you want to take part, email the NIH at [email protected].
The CDC, along with other private and public labs, is also working to develop more tests for the public.
What Do the Results Mean for You?
If you test positive for SARS-CoV-2 antibodies, it probably means you’ve had the virus. It’s also possible to get a “false positive” if you have antibodies but had a different kind of coronavirus.
A positive result might mean you have some immunity to the coronavirus. It’s too early to know how strong it is or how long it might last.
A negative result means you haven’t come into contact with the virus or you haven’t had it long enough to make antibodies. You could also have been exposed and not have antibodies. This is called a false negative.
No matter the result, if you don’t have symptoms, you don’t need follow-up. But if you do, you might have a diagnostic test to look for signs of active virus.
Because there’s a chance that test results can be wrong and because there’s so much we don’t know about the virus, it’s important to keep following official safety guidelines after your test. Stay home as much as you can, wear a cloth face mask when you’re in public, and wash your hands often.