Covid-19 antibody test: Scientists explain the most compelling reason to get tested

Six months ago, you probably didn't think much about antibodies. You maybe didn't even know what they are.

The Covid-19 pandemic changed that. Today, we covet antibodies — the tiny blood protein-based fighters sent by the immune system to help beat back the coronavirus. There’s only one way to know if you happen to be in possession of those antibodies: get tested.

However, the answer to whether you should actually go get a test isn’t as straightforward as you might think. Experts say the choice to take a test has less to do with how it will serve you personally, and more to do with how it can serve society.

Antibodies are proteins sent by the body in response to a pathogen. But not all antibodies are created equally: The prize-fighters are neutralizing antibodies. These can stop the virus from infecting other cells.

At the moment, coronavirus antibody testing only tells you if your body has encountered the virus and developed antibodies in their blood. What tests don't tell you is whether or not you have neutralizing antibodies (more on that later).

Knowing where the virus has spread and to whom is an essential step that can determine how and when governments lift quarantine measures. But that doesn't necessarily mean you should go to your doctor and demand an antibody test, Daniel Leung, an associate professor of Internal Medicine at the University of Utah, tells Inverse.

“For an individual just to know whether you’ve had the disease or not, I don’t recommend it,” Leung says. “There’s almost no use case for individual use by a consumer.”

A clear exception is if your doctor deems it necessary, he says.

It’s not just Leung taking this stance. Thomas McDade is the director of Northwestern’ Laboratory for Human Biology Research. He tells Inverse that seeking a test on your own may not be worth the resources.

“The information you will gain as an individual is mostly to satisfy your curiosity. It shouldn’t really change your behavior in any way,” McDade says.

It’s hard to square this recommendation with the publicity surrounding antibody tests (Google searches for “antibodies” and “antibody tests” have steadily risen in the past 90 days). But while antibody testing may not change your life, but it can do society a ton of good.

The most appropriate time to get an antibody test is when it's part of a larger serology study conducted under specific circumstances, both McDade and Leung explain.

When antibody tests can be useful — Scientists currently use antibody tests to conduct serosurveys. These serological surveys involve testing people not diagnosed with Covid-19 to see if they have been previously infected by its virus. Ideally, those studies take a representative, random sample of blood the population (of a city, county, state, country) and test for antibodies.

There are hundreds of serosurveys going on right now. The CDC is testing as many as 325,000 blood donor samples from 25 cities to get an idea of how much of the nation is exposed to coronavirus. Independent serosurveys have also been completed in Los Angeles and are being conducted in New York.

Those studies can reveal where past coronavirus “hotspots” have been, explains Leung. The implication of that is this information informs where additional resources can be sent to.

They can also tell you roughly how much of the population has been exposed to the virus in a certain place or help you figure out how well an intervention (like closing schools) worked to contain the virus.

“If there’s an opportunity to be part of a study, or some surveillance effort sponsored by public health authorities to know where the virus has spread in the community, that’s really important,” McDade says. “That information is only useful if lots of people participate when they’re asked."

If lots of people say no to these surveys, it can greatly hamper how useful these surveys are by skewing the sample, McDade explains. The study becomes no longer representative. Simultaneously, if lots of people who believe they've been exposed flock to get tested, then the data can also become skewed and overrepresent people who have had Covid-19.

The serological survey conducted in Santa Clara County, California was highly criticized partially because it targeted a wealthy demographic — not a representative sample of the general population. The study also drew fire for soliciting participation in an email that encouraged people with previous exposure to turn up for testing.

“You need to sample representatively among the population you’re trying to target," Leung says.

The best-case scenario is that you are selected randomly for one of these tests, says Leung.

But if you are called upon as part of a study of your neighborhood — that's what's happening to people in Miami-Dade county — you could do your local public health effort a ton of good.

Why won’t an individual antibody test change my situation? – Antibody testing, sadly, is not a get-out-of-quarantine-free card for two reasons.

The tests aren’t perfect. Due to the low prevalence of the disease, there’s a high chance that an antibody test might give you a false positive (you can find a good rundown of why that happens here.) But even if you do test positive for antibodies we don’t know if it confers lasting immunity.

In diseases like measles, immunity is permanent. For coronavirus, certain levels of antibodies “may or may not mean you’re protected,” says Leung. Whether antibodies to coronavirus indeed confer long-lasting immunity is “unanswerable right now” he continues.

Instead, Leung encourages us to think of antibody tests as a means of understanding your level of exposure — not immunity. It tells you whether you have been exposed to the virus in your neighborhood.

That’s not particularly meaningful for you as an individual, because you will still have to social distance. You still have to socially distance because antibodies don't necessarily mean prolonged immunity, and the tests aren't accurate enough to absolutely guarantee you have those antibodies in the first place.

However, it is hugely helpful for scientists conducting serosurveys, as long as those surveys have good enough sampling methods to give us useful data.

What's the difference between a PCR test and an antibody test?

A PCR test (or diagnostic test) tells you if you currently have Covid-19. These tests can impact your behavior, but not in the way you might hope.

If you have Covid-19 symptoms, speak to your doctor about getting tested. If you do test positive, the process of contact tracing can begin, explains McDade. You can self-isolate and tell everyone else you have been around that they should be quarantined too.

In an ideal world, explains McDade, every person on Earth would wake up, and take one of these rapid tests. If negative, you might about normal life. If positive, you would self-quarantine.

We have done 12 million tests for coronavirus so far, but testing capacity is still “not widely available”, says McDade. The CDC's website still notes that the first priority for diagnostic tests are hospitalized patients, front-line healthcare workers, or people who live in congregate settings where social distancing is impractical, if not impossible. That said, more and more states are beginning to allow people without symptoms to get tested as capacity increases.

“If you do take a PCR test, you’re consuming a valuable resource. I think it needs to be done judiciously and by people who suspect they may be infected,” he says.

If you do test positive and then recover, that does open up the possibility of donating plasma (which hopefully contains antibodies) to studies investigating whether we can use the blood of Covid-19 survivors as treatment. But as Leung notes, many of these studies require that you tested positive for Covid-19 before they accept donations.

The bottom line – If it’s done in the right scientific circumstances, coronavirus testing is an act of altruism. It could impact the lives of those around you if it helps you self-isolate or teaches scientists more about where the disease is spreading.

Even if you don’t reap those benefits right now, they will impact your future pandemic life.