Antibody testing shows whether you have antibodies to the virus that causes COVID-19, also called coronavirus disease 2019.

Having antibodies suggests you've either had an infection with the virus in the past or you've had the COVID-19 vaccine. Antibody testing is not used to diagnose COVID-19.

What are antibodies and why are antibody tests done?

Antibodies are proteins. The body makes antibodies when there are things in the body that shouldn't be there, such as viruses. The antibodies help clear out the virus. Having antibodies to a virus may give some protection from the disease caused by the virus for a time.

A blood test can show whether you have antibodies to the COVID-19 virus within days to weeks of having the infection or the vaccine. But antibody testing, also called serology testing, is not done routinely.

A healthcare professional might use an antibody test to diagnose complications of COVID-19. These include multisystem inflammatory syndrome, a rare condition linked to COVID-19.

If you've gotten over COVID-19, you might be able to help others who have COVID-19. If testing shows that you have a high level of antibodies, you might donate part of your blood called plasma. This is called convalescent plasma. It may help others with severe disease who have a weakened immune system.

What tests are used to diagnose COVID-19?

Two types of tests can help diagnose COVID-19.

• Molecular tests. These tests look for genetic material from the COVID-19 virus.

  Polymerase chain reaction tests, shortened to PCR tests, are molecular tests. You may also hear this type of test called an NAAT test, short for nucleic acid amplification test.

  PCR tests are more accurate than the other type of COVID-19 test, called an antigen test. You can do PCR tests at home. But they are more likely to be done by a healthcare professional and processed in a lab.

• Antigen tests. These tests look for viral proteins called antigens.

  Antigen tests also may be called rapid COVID-19 tests or at-home COVID-19 tests. These tests are useful if you need quick results.

  Antigen tests are accurate. But they're less accurate than PCR tests. If you have symptoms and an antigen test is negative for COVID-19, take another antigen test after 48 hours to get the best result.

  If you don't have symptoms and get a negative result, test again after 48 hours. If the result is still negative and you think you have COVID-19, you can test a third time after another 48 hours. Or you can get a molecular test or call your healthcare professional.

How do I get a COVID-19 diagnostic test?

In the United States, at-home COVID-19 tests are available from several sources. Free tests can be mailed to U.S. addresses, or you can buy tests in stores and pharmacies or online. The U.S. Food and Drug Administration (FDA) approves the tests. On the FDA website, you can find a list of the tests that are validated and when they expire.