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Published: May 07, 2020 10 min read
People line up outside MedRite Urgent Care.
People line up outside MedRite Urgent Care.
Cindy Ord/Getty Images

As some states ease their shutdowns, there’s a growing interest in antibody tests that can tell whether you’ve been infected with the new coronavirus — and presumably, have developed some degree of immunity to the disease.

But these tests have limitations, scientists caution. Of 14 antibody tests on the market, just three delivered consistently accurate results, according to an analysis last month by California researchers as reported by The New York Times. What’s more, experts don’t know how long immunity protection lasts with this new coronavirus.

Antibody tests are useful right now from a public health perspective, to determine what percentage of a population has been exposed to the new coronavirus. They may be less meaningful on an individual level. A test's accuracy depends on a number of factors, including its quality and the location of the person tested.

Even with an accurate test, a positive antibody result is not a free pass to re-enter daily life without restrictions, scientists say. “I would not advise people even with positive antibody tests in hand, to go back to their normal routines without thinking of social distancing,” says Daniel Larremore, assistant professor in the Department of Computer Science & the BioFrontiers Institute at the University of Colorado, Boulder.

What Is Antibody Testing?

Antibodies are proteins that attack foreign invaders in the body. The presence of antibodies for SARS-COV 2 (the virus that causes the disease COVID-19) in your blood would indicate that your immune system has battled the new coronavirus. Antibody tests are typically conducted via a blood draw after a person has recovered, whereas a nasal swab is used to diagnose an active coronavirus infection.

The accuracy of an antibody test depends on two factors: sensitivity and specificity. Sensitivity gauges whether the test can pick up on the antibody if it’s present, and the specificity gauges whether the test can distinguish SARS-COV 2 antibodies from antibodies to the six other known coronaviruses, including some of the viruses that cause the common cold.

A test that is 95% specific will give false positives 5% of the time. That is, in 5% of cases it reports the presence of antibodies when there really are none. False positives are more likely in places where the new coronavirus has not been prevalent. In a population of 1,000 people, if 1% of the people have been exposed to the coronavirus, an antibody test with 95% accuracy will give 50 false positives and just 10 true positives. So only 1 in 6, or 16.6%, of people testing positive would actually have antibodies, according to a calculation for Money by Marm Kilpatrick, a professor who studies infectious diseases at the University of California, Santa Cruz .

But the results look different in a place like New York City, the pandemic's epicenter, where preliminary data last month found that around 21% of residents had tested positive for antibodies. With that level of the population exposed, in a test with 95% accuracy, there would be 210 people who are true positives and 50 false positives, so 80.7% of those testing positive would actually have antibodies.

How Much Does an Antibody Test Cost?

The Trump administration has required group and individual health insurance to cover both COVID-19 diagnostic and antibody testing with no cost sharing to patients. So you may be able to get an antibody test for free, even if you haven’t met your plan’s deductible for the year. That said, your health insurance plan might have a list of approved facilities for antibody tests, and if you visit one that’s not on the list you might incur some out-of-pocket costs.

Quest Diagnostics recently began selling antibody testing directly to the public for $119, plus an additional lab fee of $10.30. The company uses two testing platforms: one has a specificity of 98.5-99%, while the other has a specificity of 99.4%, a spokesperson told Money in an email. The lowest number, 98.5%, gives a 1.5% chance of a false positive. That may sound small, but in areas with a low-single digit percent of the population exposed to COVID-19, it could result in as many as half of all positives being false positives, Kilpatrick says.

"We are committed to high-quality testing," a Quest spokesperson responded in an email to Money, noting that the company had performed its own quality verification tests, and that the two tests it was offering and their performance are listed on this FDA web site for authorized tests.

Where Can You Buy an Antibody Test for COVID-19

If you have private insurance, call your insurer before you get tested to be directed to an approved testing facility. Many urgent care clinics are offering antibody tests now, but you want to make sure you go to a place that your plan recognizes. Quest Diagnostics is offering tests directly to the public online, which can be purchased without going through health insurance. After you buy one, you visit a Quest facility to get your blood drawn.

The demand for antibody tests has attracted opportunists, Reuters recently reported. Some firms with no medical background have rushed into the market and are developing their own tests, taking advantage of federal oversight that’s been relaxed during the public health emergency. Consumers should investigate the accuracy of any tests they're considering, experts caution. A 95% might get you an A in school, but an antibody test with that specificity isn't the best, especially in an area with a low prevalence of the new coronavirus.

How Can States and Employers Use Antibody Tests to Safely Reopen?

Some governments are hoping that widespread antibody testing can help people get back to work, as shutdowns cripple the economy and citizen morale. Countries including the U.S. have looked into the concept of so-called “immunity passports,” which people with antibodies would carry to allow them to return to regular life without restrictions.

But there’s risk in this strategy, scientists warn. People who receive a false positive could stop social distancing and other protective measures and endanger themselves and others if they wind up getting the coronavirus.

Also, immunity passports could cause some people — such as those who can’t afford to stay home and can’t work remotely — to actively seek out infection so they could wind up with coveted antibodies, according to Alexandra L. Phelan, of the Center for Global Health Science and Security, Georgetown University Medical Center, in a recent article in The Lancet.

Even in the case of a true positive, it’s unknown how long immunity to this new coronavirus lasts. While there have been no known cases of reinfection, we could eventually see people catching the new coronavirus for a second time as immunity protection wanes, Kilpatrick says. His best guess, based on data from other coronaviruses, is that immunity against COVID-19 might last for between one and four years after you get infected. If a vaccine is developed within that time frame, people who had the virus once may never get it again.

A useful way to employ the most accurate antibody tests right now might be by high-risk industries, Kilpatrick says. (The pharmaceutical company Roche recently announced a test with a specificity of greater than 99.8% and sensitivity of 100%.) For example, a hospital might offer the tests anonymously to all its nurses, and allow those with antibodies to volunteer in the COVID-19 ward, knowing that some of those results might be false positives. Or a grocery store could test all employees and allow those with antibodies to volunteer for placement at the checkout counter, whereas those without might prefer to work in the stockroom where they have no contact with the public.

The best way to employ immunity passports would be on the basis of vaccination, Kilpatrick says. Of course, there's no way to do that now, since a vaccine for COVID-19 doesn't exist. But if scientists succeed in developing one, an immunity passport based on that would reward people for protecting themselves and their communities.

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