Scientist marking a test tube in her lab.
Scientist marking a test tube in her lab.
Scientist marking a test tube in her lab.

Coronavirus Testing: What Is an Antibody Test?

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There are at least two kinds of tests for the coronavirus, a viral test and an antibody test. The viral test looks for evidence of the coronavirus in your respiratory system. A positive viral test means that you have SARS-CoV-2, the type of coronavirus that causes COVID-19.  You may or may not have symptoms of the infection.

A different kind of test looks for antibodies, which show you have been exposed to the coronavirus and that your immune system responded to it. Ben Larman, a Johns Hopkins researcher in immunopathology, helps you understand more about antibody tests and how they might be used in the COVID-19 pandemic.

What are antibodies?

Antibodies are proteins created by the body’s immune system to fight a particular virus, such as SARS-CoV-2. Antibodies are very specific for their intended target. So antibodies directed toward one virus would not protect the body from another. In other words, if you have had the measles, your body has antibodies for the measles virus, but measles antibodies will not protect you from catching the coronavirus.

What is an antibody test?

Since antibodies are abundant in the bloodstream, the antibody test is typically a blood test. Antibodies are also present in saliva, and many salivary antibody tests are now on the market. These tests, however, may not work as well as blood tests. A health care professional takes a sample of your blood and sends it to a lab to look for COVID-19 antibodies. This is different from the viral COVID-19 test, which involves swabbing the inside of your nose and the back of your throat to collect saliva and respiratory fluids to look for evidence of the virus itself.

Is the antibody test better than the viral test?

They are two different tests, which provide complementary information, so it’s best to go with what your doctor recommends. The antibody test can be useful to tell if a person has been infected with the coronavirus in the past, even if they did not have any symptoms. Certain kinds of antibody tests may provide more information about your risk for re-infection. However, this question of whether people who have SARS-CoV-2 antibodies are protected against future infection with the virus is still a very active area of investigation.

Should I get an antibody test?

Talk to your doctor, who can help you decide if getting an antibody test is appropriate. Doctors might recommend an antibody test if you develop symptoms consistent with COVID-19 but your viral test is negative, or if you had symptoms of COVID-19 earlier this year but didn’t get tested for it. If your antibody test is positive, you might be able to participate in research studies working to understand the effects of the coronavirus on people’s health. Also, if you have the antibodies in your blood, you might be able to donate plasma to help another person who is very sick with COVID-19.

How do I get an antibody test?

Check with your doctor or local health department, since the availability of antibody tests varies depending on where you live. Some commercial laboratories also offer them for a fee.

Antibody test results: What do they mean?

Negative (antibody test) results mean that either (1) you have not been exposed to the coronavirus, or (2) you were exposed to the virus, but at the time of your test it was too soon for your body to produce antibodies or the level of antibodies present at the time of the test were below the test’s limit of detection. It takes one to three weeks after exposure to the coronavirus to develop antibodies. It is also possible that many weeks or months following an infection, the antibody test may be negative as well. For some infections, antibodies decrease over time, but if the individual is exposed again to the same infectious virus, the body gears up and rapidly produces the needed antibody defenses. We are not yet sure whether SARS-CoV-2 antibodies protect the person against reinfection with the virus in the future.

Positive (antibody test) results mean that you have likely encountered the coronavirus at some point. It is still essential to protect yourself by taking precautions such as physical distancing, hand washing and mask wearing until more is known about immunity to this virus, and how long people infected with it remain contagious to others.

Does a positive antibody test mean I am immune to COVID-19?

This is a question that researchers are eager to answer. In some diseases, the presence of antibodies means you are immune, or protected against future infection. Your body has learned to recognize that virus and has created antibodies to fight it.

For some viruses, such as the one that causes measles, your immunity is more or less permanent. For others, the immunity may fade over time. Researchers hope that having some antibodies to the coronavirus might protect you from a more severe case of COVID-19.

More research will help clarify the relationship between having antibodies and being immune or protected from future SARS-CoV-2 infection.

Assume you can catch — and spread — coronavirus

Since one of the most puzzling things about this new coronavirus is how differently it affects individuals, testing is the best way to determine whether or not you have COVID-19.

Whether or not your antibody test is positive or negative, you should remember that you might still be able to catch COVID-19 or unknowingly spread the disease to someone else if you carry the coronavirus, regardless of whether you have any symptoms.

That is why, regardless of your antibody status, mask wearing in public is essential to preventing spread of COVID-19, along with physical distancing and hand hygiene. If you are newly symptomatic, getting a viral test would be important to determine if new infection has occurred.

Scientist carefully insets a pipette into a test tube.

Coronavirus (COVID-19)

What you need to know from Johns Hopkins Medicine.

Published July 31, 2020