Overview

COVID-19 antibody testing, also known as serology testing, is a blood test that's done to find out if you've had a past infection with SARS-CoV-2, the virus that causes coronavirus disease 2019 (COVID-19). An antibody test can't determine whether you're currently infected with the COVID-19 virus.

Antibodies are proteins produced by your immune system in response to an infection. Your immune system — which involves a complex network of cells, organs and tissues — identifies foreign substances in your body and helps fight infections and diseases. After infection with the COVID-19 virus, it can take two to three weeks to develop enough antibodies to be detected in an antibody test, so it's important that you're not tested too soon.

Antibodies may be detected in your blood for several months or more after you recover from COVID-19. Although these antibodies probably provide some immunity to the COVID-19 virus, there's currently not enough evidence to know how long the antibodies last or to what extent past infection with the virus helps protect you from getting another infection. Though rare, there are some confirmed and suspected cases of reinfection. Studies on COVID-19 antibodies as well as other components of the immune system are ongoing to learn more about immunity.

Antibody tests may detect certain types of antibodies related to the COVID-19 virus:

• Binding antibodies. These widely available antibody tests detect whether you've developed any antibodies in response to a COVID-19 infection. But they don't indicate how extensive or effective your immune response is.
• Neutralizing antibodies. Not yet widely available, a newer and more sensitive test detects a subgroup of antibodies that may inactivate the virus. This test can be done after you test positive for binding antibodies. It's another step toward finding out how effective your antibodies are in blocking the virus to help protect you from another COVID-19 infection.

Why it's done

Antibody testing for COVID-19 may be done if:

• You had symptoms of COVID-19 in the past but weren't tested
• You're about to have a medical procedure done in a hospital or clinic, especially if you've had a positive COVID-19 diagnostic test in the past
• You've had a COVID-19 infection in the past and want to donate plasma, a part of your blood that contains antibodies that can help treat others who have severe cases of COVID-19

If a child is sick and the doctor suspects multisystem inflammatory syndrome for children (MIS-C), antibody testing may be ordered to help diagnose MIS-C. Many children with MIS-C have antibodies to COVID-19, indicating past infection with the coronavirus.

If you're interested in having a COVID-19 antibody test, contact your doctor or your local health department. Whether or not you're eligible for testing may depend on the availability of tests in your area and local or state health department guidelines.

Risks

Results of COVID-19 antibody tests may not always be accurate, especially if the test was done too soon after infection or the test quality is questionable. Many different manufacturers rushed to put antibody tests on the market with little oversight. Now the U.S. Food and Drug Administration posts data online about the performance of certain antibody tests.

COVID-19 antibody testing could lead to false-positive or false-negative test results:

• False-positive result. The test result is positive, but you actually don't have antibodies and you did not have an infection in the past. A false-positive result could give you a false sense of security that you're protected from getting another COVID-19 infection — and even with a true positive result, immunity is questionable.
• False-negative result. You have antibodies to the COVID-19 virus, but the test does not detect them. Or you're tested too soon after infection and your body has not had time to develop antibodies.

How you prepare

Your doctor or testing center will provide instructions for where to go for testing and how the test will be done. Plan to wear a face mask to and from the testing center. Anyone who comes with you will need to wear one, too.

What you can expect

To conduct an antibody test for COVID-19, typically a health care professional takes a blood sample, usually by a finger prick or by drawing blood from a vein in your arm. Then the sample is tested in a lab to determine whether you've developed antibodies against the COVID-19 virus.

COVID-19 antibody test results may be ready the same day as your test at some sites. Other places may have to send test samples out to a lab for analysis, so results may not be available for a few days.

Results

COVID-19 antibody test results could be:

• Positive. A positive test means you have COVID-19 antibodies in your blood, which indicates past infection with the virus. It's possible to have a positive test result even if you never had any symptoms of COVID-19. False-positive test results can occur. It may be that the test detected antibodies to a coronavirus closely related to the COVID-19 virus or that the test quality was flawed.
• Negative. A negative test means that you have no COVID-19 antibodies, so you probably were not infected with the COVID-19 virus in the past. Because it takes time for antibodies to develop, false-negative test results can happen if the blood sample is collected too soon after your infection started. In some cases, the test may be flawed.

People who have had COVID-19 or tested positive for antibodies should not assume they're protected from getting a COVID-19 infection again. Researchers are trying to determine if antibodies provide some immunity to the COVID-19 virus, what the level of protection is and how long immunity may last.

Until more is known, even if your test results show that you have COVID-19 antibodies, continue to take precautions — including wearing a face mask in public, frequently washing your hands and practicing social distancing — to avoid the risk of spreading the virus.