What is it?

An antibody test (also called a serology test) looks for antibodies in the blood. These are proteins that your immune system makes, usually after you're exposed to germs like viruses or bacteria or after you get a vaccine. Antibodies work to fight illness.

A COVID-19 antibody test looks for antibodies to SARS-CoV-2, the virus that causes COVID-19. If you test positive for these antibodies, it could mean that you already had COVID-19. If you’ve received the COVID-19 vaccine, it’s not yet known how this will affect the antibody test results.

Why is it done?

This test can be used to diagnose a past infection with the virus that causes COVID-19. This test is not useful for finding out if you have COVID-19 now because it can take weeks for your body to make enough antibodies to show up on the test. Other testing that directly detects the virus, like the COVID-19 viral test (or nucleic acid test) is used to diagnose new COVID-19 infections.

Antibody testing is useful because:

• A positive result shows that you've already had COVID-19. But even if you have the antibody against SARS-CoV-2, you’re not considered immune to the virus. Keep doing things like wearing a mask, washing your hands often, and practising physical distancing.
• It could show who hasn't had COVID-19. Even though a negative result shows you haven’t been infected, you’re still at risk. Keep taking steps to protect yourself from the virus.
• It helps experts who are tracking COVID-19 learn more about the virus and how it spreads.

Talk to your doctor about what the test results mean for you.

How do you prepare for the test?

You don't need to do anything to prepare for this test. But be sure to follow any instructions your healthcare provider gives you.

How is it done?

This is a blood test. A health professional will use a needle to take a sample of blood from your arm.

What do your results mean?

The result is either positive or negative.

A positive result means antibodies to SARS-CoV-2 were found. You probably already had COVID-19 or had a COVID-19 vaccine. But:

• You could get a "false-positive" result. The test might show that you have COVID-19 antibodies when you don't. The test may find antibodies that formed in response to another type of coronavirus.
• It's not certain that having these antibodies will protect you from getting COVID-19 again. Even if there is some protection with these antibodies, it's not clear how long the protection lasts. Whether your antibody test result is positive or negative, you’re still at risk for COVID-19.

A negative result means that these antibodies were not found.

• You could get a false-negative result. It takes a while after you're infected for your immune system to make antibodies. You could have a negative result but be infected with COVID-19. You'd need a different test (viral test) to know if you have COVID-19 now.