I've heard about antibody testing for COVID-19. What is antibody testing? Is it the same as testing to diagnose COVID-19?

Answer From William F. Marshall, III M.D.

With all the talk about coronavirus disease 2019 (COVID-19) testing in the news, it's not surprising that there's confusion about tests and how they differ. Antibody testing determines whether you had COVID-19 in the past and now have antibodies against the virus. A test to diagnose COVID-19 determines if you currently have the disease. Here's what you need to know about testing.

When is antibody testing done and why is it important?

Antibody testing, also known as serology testing, is usually done after full recovery from COVID-19. Eligibility may vary, depending on the availability of tests. A health care professional takes a blood sample, usually by a finger prick or by drawing blood from a vein in the arm. Then the sample is tested to determine whether you've developed antibodies against the virus. The immune system produces these antibodies — proteins that are critical for fighting and clearing out the virus.

If test results show that you have antibodies, it indicates that you were likely infected with COVID-19 at some time in the past. It may also mean that you have some immunity. But there's a lack of evidence on whether having antibodies means you're protected against reinfection with COVID-19. The level of immunity and how long immunity lasts are not yet known. Ongoing studies will eventually reveal more data on this.

The timing and type of antibody test affects accuracy. If you have testing too early in the course of infection, when the immune response is still building up in your body, the test may not detect antibodies. So antibody testing is not recommended until at least 14 days after the onset of symptoms. The U.S. Food and Drug Administration (FDA) authorized specific antibody tests, but tests with questionable accuracy are still on the market.

Another benefit of accurate antibody testing is that people who've recovered from COVID-19 may be eligible to donate plasma, a part of their blood. This plasma could be used to treat others with severe disease and boost the ability to fight the virus. Doctors call this convalescent plasma.

What tests are used to diagnose COVID-19?

The FDA approved these types of tests for diagnosing a COVID-19 infection:

  • PCR test. Also called a molecular test, this COVID-19 test detects genetic material of the virus using a lab technique called polymerase chain reaction (PCR). A fluid sample is collected by inserting a long nasal swab (nasopharyngeal swab) into your nostril and taking fluid from the back of your nose or by using a shorter nasal swab (mid-turbinate swab) to get a sample. In some cases, a long swab is inserted into the back of your throat (oropharyngeal swab), or you may spit into a tube to produce a saliva sample. Results may be available in minutes if analyzed onsite or a few days — or longer in locations with test processing delays — if sent to an outside lab. PCR tests are very accurate when properly performed by a health care professional, but the rapid test can miss some cases.
  • Antigen test. This COVID-19 test detects certain proteins in the virus. Using a long nasal swab to get a fluid sample, antigen tests can produce results in minutes. Because these tests are faster and less expensive than PCR tests, antigen tests may be more practical to use for large numbers of people. A positive antigen test result is considered accurate when instructions are carefully followed, but there's an increased chance of false-negative results — meaning it's possible to be infected with the virus but have a negative result. Depending on the situation, the doctor may recommend a PCR test to confirm a negative antigen test result.

Certain COVID-19 test kits authorized by the FDA allow you to collect the sample at home and then send it to a lab to be analyzed. One at-home COVID-19 test that's authorized by the FDA provides fast results at home. This nasal swab test requires a doctor's prescription. At-home PCR tests are likely more accurate than at-home antigen tests are, but a sample collected by a trained professional may be more reliable. The FDA warns consumers that some scammers are promoting unauthorized COVID-19 tests, so only get a test that's authorized by the FDA.

What do I do if I'm interested in a COVID-19 diagnostic test or an antibody test?

You can contact your local or state health department or visit the department's website for information on testing. If you have symptoms of COVID-19, contact your doctor to discuss your situation and find out how to prepare for your appointment before seeking a COVID-19 test in person.

A COVID-19 diagnostic test is recommended if you have symptoms or you've had close contact with someone who tests positive for the COVID-19 virus or is suspected of having the virus. To get antibody testing, you have to be fully recovered from COVID-19. But in some communities, people who never had symptoms of COVID-19 may be able to get tested. Some have positive results, meaning they likely were infected by the COVID-19 virus at some time.

Access to either test depends on where you live, test availability and whether you're viewed as eligible. In the U.S., collaborative efforts to make more tests available are ongoing. The nationwide goal is to test more people as more tests become available.

How can diagnostic and antibody testing help reduce the spread of COVID-19?

With COVID-19 diagnostic testing, people who test positive and have symptoms can get care earlier. Contacts can be traced and self-isolation or quarantine started sooner to help stop the spread of the virus.

But no COVID-19 test is 100% accurate. It's possible to test negative yet actually be infected (false-negative result) or to test positive and not be infected (false-positive result). So it's vital to continue to follow COVID-19 guidelines from the Centers for Disease Control and Prevention, such as social distancing and wearing a mask when appropriate, until further notice.

Results of antibody tests indicate how many people had COVID-19 and recovered, including those who didn't have symptoms. This aids in determining who might have immunity, though to what extent and for how long is not yet known.

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William F. Marshall, III M.D.

Nov. 24, 2020