Everything We Know About Coronavirus Antibody Tests

Antibody tests help public-health officials better understand how much a virus has penetrated society. Photo: Omar Marques/Getty Images

Much of the discussion around coronavirus testing concerns the type of test that requires a long swab to go deep in your nose so you can find out, eventually, if you have the virus. These tests are helpful at diagnosing patients but less helpful at assessing the spread of the virus. For that, experts have called for antibody tests. What is an antibody test and why would it help? Here’s what we know:

What is an antibody test?

Antibody tests, also known as serological tests, look for antibodies in a patient’s blood. People who have gotten sick and recovered from COVID-19 have these antibodies, as do those who had the virus without ever experiencing symptoms. Some antibody tests, which require a quick finger prick, can return results in as soon as 15 minutes.

Can an antibody test detect an active coronavirus infection?

An antibody test is not meant to detect an active infection, as it can take ten days or more for coronavirus antibodies to become detectable in a blood sample after someone has recovered.

As of the end of April, only two kinds of tests are being used to detect active COVID-19 infections, both by looking for the virus’s genetic material in samples obtained by either a PCR (nose swab) test or, in places where it is available, a saliva test.

Why are antibody tests helpful?

As Dr. Anthony Fauci said on April 10, these tests will help health officials better understand how many people have been infected with COVID-19.

“As we look forward, as we get to the point of at least considering opening up the country as it were, it’s very important to appreciate and to understand how much that virus has penetrated this society,” Fauci said. “It’s very likely that there are a large number of people out there that have been infected, have been asymptomatic, and did not know they were infected.”

The tests could detect people with some amount of immunity to COVID-19, which, theoretically, might allow them to reenter the workforce without concern that they’ll get sick or infect anyone else, but there is still an active debate about that. The tests could also indicate who has not been exposed to the virus and remains at risk.

From a public-health perspective, results of these tests could change, or confirm, what is known about the virus’s spread, particularly since the U.S. has struggled to increase its testing capacity, making it likely that a still-unknown number of coronavirus cases have gone undetected. More than a million confirmed cases of COVID-19 have been detected in the U.S. so far; if antibody tests reveal that, say, 4 million people have actually had the virus, that would be very valuable information for public-health officials.

Does the presence of coronavirus antibodies confer immunity?

Not necessarily. The presence of antibodies only indicates that someone has had COVID-19, but it does not guarantee that person can’t become reinfected with the disease. And even if they are protected, it’s not clear how long that protection will last.

On April 24, the WHO cautioned that “no study has evaluated whether the presence of antibodies to SARS-CoV-2 confers immunity to subsequent infection by this virus in humans,” and recommended against the use of the tests as the basis for any kind of immunity certificate.

All that being said, most experts agree that the antibodies produced by having and recovering from COVID-19 will confer some amount of immunity — it’s just not clear how much. Experts warn that a positive antibody test may result in a false sense of security. Here’s a useful explanation from the Atlantic’s James Hamblin on the reliability of antibodies in helping fight a coronavirus infection:

Usually antibodies work very reliably, but in some cases they barely help, and in certain diseases having some antibodies is worse than having none. This is known as immune enhancement, a phenomenon that may or may not prove relevant with this coronavirus; it is worth keeping in mind when people suggest that antibody tests are currently painting a clear picture of who is totally protected from the disease.

How accurate are coronavirus antibody tests?

There are some very valid concerns about the accuracy of the currently available coronavirus antibody tests, as well as their value, since no test is able to determine if or when someone with antibodies could be reinfected with the coronavirus.

A recent study, which has not been peer reviewed, evaluated 14 coronavirus antibody tests on the market and found that only three provided consistently accurate results. According to the New York Times, those three tests were able to detect the antibodies no more than 90 percent of the time, and only one of the tests never delivered a false positive, while the other two did not deliver false positives 99 percent of the time.

Four tests delivered false positives anywhere from 11 to 16 percent of the time. Overall, the false positives were less likely when the blood sample was taken longer after the patient recovered. (More specifically, the test results were less reliable when detecting the transient initial antibody IgM than they were when detecting the subsequent antibody IgM.)

A House panel recently called attention to the threat of inaccurate tests, and on May 4, the FDA announced that all companies that sell antibody tests must submit data proving their accuracy within ten days, or else they would be forced to remove their test from the market.

The other issue is that antibody tests may be far more valuable to public-health researchers than they are to individuals or for policy-makers at a local level. Florian Krammer at the Icahn School of Medicine at Mount Sinai in New York offered this warning about false positives to the Times:

Dr. Krammer said false positives are less of an issue for assessing how widely the virus has spread in the population. If a test has a known false-positive rate, scientists can factor that into their calculations, he said.

But false positives become dangerous when making policy and personal decisions about who can go back to work. “You don’t want anybody back to work who has a false positive — that’s the last thing you want to do,” Dr. Krammer said.

What antibody tests are available, and what has the FDA been doing to make sure they work?

As of the week of May 4, only 12 companies’ antibody tests have been verified and authorized by the FDA. On April 18, the FDA said it would partner with the NIH and CDC to begin validating tests that manufacturers voluntarily submit. But under an emergency policy meant to help rapidly expand coronavirus testing in the U.S., the FDA allowed 160 antibody tests to hit the market without having to undergo government review — provided the companies selling the tests follow a few rules, which the FDA has already acknowledged not all of them have done. How much policing the agency subsequently did has also come under scrutiny, as NBC News reported on April 29:

It’s unclear how much policing the FDA is doing when test manufacturers overstate the applicability of their tests. House Oversight Committee staff members found that the FDA has no dedicated staff focused on enforcement regarding antibody tests. The same FDA officials who review applications for emergency authorization are given the double duty of monitoring the market and acting on violations.

FDA Commissioner Stephen Hahn has said “we have and will” take action “such as issuing Warning Letters” telling companies to stop making false claims. But the FDA didn’t respond to inquiries about how many letters have been issued or whether the agency has pursued any civil or criminal actions against violators.

Then on May 4, the FDA directed all companies selling antibody tests to submit data within ten days proving they are effective or face removal from the market. It’s not yet clear how many of the 160 tests companies have made available will soon be left in light of that requirement — assuming the FDA effectively enforces it. According to the FDA, more than 100 companies have begun working with the agency to submit data.

Meanwhile, state-level protocols also apply. For instance in New York, strict regulations have likely limited the availability of ineffective tests.

Two of the biggest U.S. lab testing companies, LabCorp and Quest Diagnostics, have announced that they are offering nationwide coronavirus antibody testing. Quest’s test is being offered direct to consumers at its patient service centers. LabCorp is only testing blood samples collected by health-care providers; the test must be ordered by a physician and can be done at a LabCorp service center or at a partnering Walgreens. Neither company has received official FDA authorization for their tests, and both companies have warned that a positive antibody test does not confer immunity.

How can you get an antibody test?

Anyone who is currently experiencing symptoms of the coronavirus should contact their health-care provider, as antibody tests cannot be used to diagnose active infections. Anyone who believes they have had the virus and wants to get an antibody test, whether they have previously tested positive for COVID-19 or not, have a few options.

Through a health-care provider
Any health-care provider can likely arrange for an antibody test, and if ordered by a physician, it’s likely most insurance plans would cover the cost. LabCorp is offering antibody tests with a physician’s referral at its more than 2,000 service centers and 100 partnering Walgreens pharmacy locations.

Quest Diagnostics
Quest is selling its $119 direct-to-consumer COVID-19 Immune Response test via its QuestDirect website. A physician’s approval is not required. When ordering the test, customers will need to make an appointment to have their blood drawn at one of the companies 2,200 local service centers, as well as affirm that they have not had any coronavirus symptoms in the past ten days.

The company says its test meets FDA standards, and that results should be available in a few days.

PM Pediatrics
On May 4, the pediatric urgent care company announced that it was offering antibody testing, without the need to confirm symptoms, at 36 locations around the country, and would test entire households. There would be no copay for patients with insurance, and the tests cost $65 for anyone self-paying.

City MD (in New York City only)
The New York walk-in urgent-care chain City MD announced on April 28 that it would offer antibody testing to anyone who has had coronavirus symptoms at least 14 days prior to seeking the test, or if they believe they have come into contact with someone who had the virus.

Free testing programs
On May 3, Jersey City announced that all residents of the city will able to get free tests for either an active coronavirus infection or antibodies, without a doctor’s referral, at a drive-through testing site in the city. Bergenfield is also providing free tests to its residents, regardless of symptoms.

How can people with coronavirus antibodies help people with active infections?

Some health-care providers have already set up programs to test patients who have recovered from confirmed cases of COVID-19 in an effort to collect plasma, which can be used for convalescent treatment of people fighting active coronavirus infections. In the New York City area, Mount Sinai Hospital, Montefiore Medical Center, and the New York Blood Center are asking recovered COVID-19 patients to donate plasma. New Yorkers can also email for possible testing.

The National COVID-19 Convalescent Plasma Project has contact information for those interested in donating plasma in California, Colorado, Florida, Houston, Illinois, Maryland, Minnesota, North Carolina, Ohio, Philadelphia, St. Louis, Washington, D.C., and Wisconsin.

How does antibody testing differ from antigen testing?

An antigen test is a kind of diagnostic test used to rapidly detect active infections by checking a sample for proteins attached to the virus. As of the end of April, no coronavirus antigen tests were available, but several were in development. There have also been some concerns about the accuracy of antigen tests.

This post has been updated throughout to reflect newly available information.

Everything We Know About Coronavirus Antibody Tests