How antibody tests work and could help fight the coronavirus

Questions about the tests’ accuracy still need to be answered

Coronavirus blood samples

Coronavirus antibody tests look for signs in the blood that someone has had an infection and recovered, and could take only a finger prick. But questions about the tests’ accuracy remain.

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The United Kingdom has ordered 3.5 million antibody tests, which would show whether someone has been exposed to COVID-19. Such tests, which just take a drop of blood, could help reveal people who have been exposed to the virus and are now likely immune, meaning they could go back to work and resume their normal lives.

Antibodies are proteins that the body’s white blood cells produce to fight an infection. They bind to a virus, preventing it from infecting a cell, and can remain in blood long after the infection clears. Antibody tests are commonly used to test for exposure to other viruses.

Science News spoke with David Weiner, director of the Vaccine and Immunotherapy Center at the Wistar Institute in Philadelphia, and Charles Cairns, dean of the Drexel University College of Medicine, about how antibody tests work and what are some of the challenges of developing the tests.

The following responses have been edited for brevity and clarity.

SN: What does an antibody test do?  

Cairns:  Antibody tests look to see if someone has been exposed to a specific antigen, like a virus. The British tests are designed to work in one of two ways. They either detect human antibodies in blood using an antigen designed to be similar to a feature of the virus. Or conversely, the test detects the virus in blood using a [human-made] antibody designed to trap the virus.

SN: How is it different from tests that diagnose an infection?

Cairns:  Diagnostic tests are using RT-PCR tests (SN: 3/6/20). You take a nasal swab (or sputum sample) that identifies the specific viral RNA from the COVID-19 virus.  It’s the gold standard to see if you are actively infected.

The antibody tests are quick — a prick of blood and you get a yes/no answer. You’ve had COVID-19 or you haven’t.

SN: Why do antibody tests matter?

Weiner:  People who have recovered won’t have RT-PCR positive tests, as they’ve already cleared the virus. Those who are recovered, those antibodies protect them from reinfection. (It’s still unclear, however, how long that immunity might last.)

We’re now screening to find people who are positive [and have a current infection]. But it’s important to find people who have recovered and who are unlikely to be reinfected, so they can go out and be the buffers for the rest of us. That’s how herd immunity develops (SN: 3/24/20).  Antibody tests also give us a better idea of how many people have been infected and how many have recovered, so we can start to move forward.

SN: Who should be prioritized for getting this test?

Weiner: I think the first responders and health care workers should be screened first, because it’s very important for them [to feel confident] to get back out on the front line.  

SN: What is the status of providing widespread antibody testing in the United States?

Cairns: There are lots of people working on [antibody] tests [in the United States]. But you’re trying to replicate a gold standard — the RT-PCR test — with a test that uses smaller amounts of sample, is faster and is available outside the lab. 

The big question is: Does a positive response for the antibodies mean that person is actively infected, or that they have been infected in the past? The tests need to be accurate, and avoid both false positives and false negatives. That’s the challenge. 

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