What Is a Coronavirus Antibody Test—and Why Testing Matters

Citing false positives, CDC says antibody tests should not be used to make decisions about returning to work or school.

Just a few weeks ago, experts thought widespread antibody testing would be the key to lifting stay-at-home orders. An antibody test, also known as a serology test, uses a blood sample to determine whether someone has been been infected by COVID-19 and, potentially, developed some degree of protection from reinfection. But even as testing capacity continues to ramp up, states and communities are already easing restrictions on social and business activities.

Meanwhile, another hitch has come to light: these tests may be inaccurate half of the time, according to the US Centers for Disease Control and Prevention. If a test with 95% specificity (meaning it can accurately detect 95% of people without the disease) were used in a community where the actual infection rate is low, say, just 5%, almost half of all positive tests would be false positives, CDC explains.

Serology testing can be useful on a population level to determine, for example, which communities may have experienced higher rates of infection and possibly developed some herd immunity, or which groups of people are at higher risk of infection. For individuals, though, these tests are of limited value. Says CDC: "serologic tests should not be used at this time to determine if an individual is immune."

Here's what you should know about antibody testing.

What Is Antibody Testing?

Antibodies are human proteins produced by the body to fight infection. “During an infection, the immune system custom-designs antibodies to recognize a specific part of the virus,” Craig Wilen, MD, an assistant professor in laboratory medicine and immunobiology at Yale University, tells Health.

In the case of the new coronavirus, the most important antibodies target the viral spike protein, which is the protein that binds to the human cell receptor and allows the genetic material of the virus to enter human cells and begin infection. “There are two types of virus-specific antibodies that are relevant for testing,” Dr. Wilen says. “The first is called immunoglobulin M (IgM). This antibody is made earlier in the period of infection and then goes away. The second is called immunoglobulin G (IgG), which is made after IgM, but lasts much longer.”

While there is some variability in timing between people, IgM lasts for weeks to months and is detectable after a week or two, whereas IgG lasts for decades and is detectable days to about one week after IgM. By detecting the different antibodies in a person’s blood, doctors are able to “date” when they had—or still have—the infection.

If My Test Comes Back Positive, Am I Immune?

Unfortunately, scientists don’t yet know whether a positive test means you’re immune or how long such protection may last. “There have been reports of patients getting re-infected with the new coronavirus,” Dr. Wilen says. “It is also known that for many seasonal coronaviruses which cause the common cold, the antibodies go away over time which enables people to get re-infected.”

The World Health Organization addressed this issue at a news briefing in Geneva on April 13. "One would expect that a person who generates a full-blown immune response with detectable antibodies should have protection for a period of time," Dr. Mike Ryan, executive director of WHO's emergencies programs, said, per NPR. "We just don't know what that period of time is. We would expect that to be a reasonable period of protection, but it is very difficult to say that with a new virus."

Dr. Wilen suspects that even if the new coronavirus antibodies went away, that past infection would confer a survival advantage to future re-infection. “This is a general principle of the adaptive immune system,” he explains.

Are Tests Available—and What Does Testing Involve?

The Food and Drug Administration (FDA) recently granted emergency use of a test developed by Cellex Inc. for detecting coronavirus antibodies. It was the first antibody test to be cleared for use in the US. To date, the FDA has authorized 13 such tests. But Politico reported that the agency has allowed many other diagnostic developers to flood the market with tests that haven't been reviewed, and now there is a concern that some tests are yielding "too many false positive results."

Meanwhile, the CDC has already started carrying out its own antibody testing to help determine how many people have been infected with the virus, including those who never displayed any symptoms, reported Politico. On April 10, the National Institutes of Health announced that it would test 10,000 healthy volunteers from around the country for the presence of antibodies. People can participate in at-home blood sampling using a kit and mail the sample to the NIH lab for analysis.

Typically, such tests involve taking blood from the arm or using the finger-stick method, in which a lancet used to prick a person's finger to obtain a small quantity of capillary blood. “The amount of blood needed depends on the specific test method,” Dr. Wilen says. Either way, it should be quick and simple.

Consumers who want to know whether they have been exposed to the virus now now have new options. Quest Diagnostics announced that people can buy its testing service online and then schedule an appointment for a blood draw. Results are promised in a day or two. Separately, LabCorp said it will offer antibody testing to people who receive an order for the test from their doctor. The blood tests will be administered through its patient service centers and Walgreens locations.

How Reliable Are Antibody Test Results?

In other parts of the world, some governments have expressed concern that antibody tests are not proving to be as reliable as the nasal swab tests that look for current infection. On April 8, the UK government announced that antibody tests won’t be available until at least May because none of the tests tried so far had been accurate enough. And there's some uneasiness that the same thing could happen in the US.

It’s crucial that the coronavirus antibody testing process isn’t rushed, Gary W. Procop, MD, the chair of American Society for Clinical Pathology’s Commission on Science, Technology and Policy, and vice chair and director of virology at Cleveland Clinic, tells Health. If the tests aren’t sensitive or specific enough, it could result in false positives, which suggest a person has immunity who doesn’t, and false negatives, which fail to detect immunity.

“Both of these are dangerous and hamper the efforts to get Americans safely back to work,” Dr. Procop says. “We need to follow the lead of our colleagues in the UK and not implement tests that have not been thoroughly vetted and may do more harm than good.”

At this time, says CDC, these tests should not be used to make decisions about returning people to the workplace or grouping people in congregate settings, such as schools, dormitories, or correctional facilities.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.

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