How do COVID-19 antibody tests differ from diagnostic tests?

Date Updated: 02/01/2022

Answer Section

With all the talk about coronavirus disease 2019 (COVID-19) testing, it's not surprising that there's confusion about tests and how they differ. Antibody testing is usually done to see whether you had COVID-19 in the past and now have antibodies against the virus that causes COVID-19. A test to diagnose COVID-19 determines if you currently have the virus that causes COVID-19. 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 that causes COVID-19. 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 can mean that you have been infected with the COVID-19 virus in the past or you have antibodies after being vaccinated. It may also mean that you have some immunity. However, having antibodies may not mean you're protected against reinfection with COVID-19. But they can help prevent severe disease. The level of immunity and how long immunity lasts aren't yet known and continue to be studied.

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 2 to 3 weeks after your symptoms started. 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:

RT-PCR test. Also called a molecular test, this COVID-19 test detects genetic material of the virus using a lab technique called reverse transcription polymerase chain reaction (PCR). A fluid sample is collected with a nasal swab or a throat 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. RT-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 nasal swab to get a fluid sample, antigen tests can produce results in minutes. Others may be sent to a lab for analysis.

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 health care provider may recommend a RT-PCR test to confirm a negative antigen test result.

The FDA authorized for emergency use certain at-home COVID-19 test kits, including one that tests for both COVID-19 and the flu (influenza). You can collect your own sample of nasal fluid or saliva at home and then send it to a lab to be rapidly analyzed. Some of these tests require a doctor's prescription.

Some COVID-19 tests provide fast results at home without sending the sample to a lab. You can buy some antigen tests over the counter with no prescription needed. But antigen tests are not considered as reliable as PCR tests. If you test negative, taking the test a second time can help ensure that your test results are accurate.

The accuracy of these tests varies. A negative test does not completely rule out having the COVID-19 virus. Only get an at-home test that's authorized by the FDA or approved by your local health department.

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 COVID-19 symptoms or you've been exposed to someone with COVID-19, you should get tested. If you have COVID-19 symptoms, contact your health care provider to review your symptoms and ask about testing before you go in, so staff can prepare for your visit.

Access to either test can depend on where you live, test availability and whether you're viewed as eligible.

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. Self-isolation or quarantine can be started sooner to help stop the spread of the COVID-19 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 precautions, such as washing hands regularly, avoiding crowds and wearing a mask when appropriate. If you feel sick, stay home and away from others.

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.