Coronavirus Antibody Test: What You Need to Know

As states across the country weigh options for reopening after weeks of stay-at-home orders, antibody tests have emerged as a potential pathway on how — and when — to do it.

But there are many caveats, as a recent study found that many of the antibody tests available currently provided inaccurate results.

Antibody tests look for signs in the blood that a person has been exposed to the novel coronavirus. Knowing who has been exposed, along with how many people have been, may help to better understand the spread of the virus. This is especially important as studies continue to show that significant percentages of all coronavirus carriers — in some studies, up to half — show no symptoms at all.

The Food and Drug Administration in the United States said this month that “the question of when we can return to work and resume our normal activities is one of the most critical issues facing our nation. Antibody tests — also known as serological tests — may have the potential to play a role in this complex calculation.”

But should you get one? Can you get one? What do they actually tell us? Here’s everything you need to know.

The antibody test, also called a serology test, looks at whether your body has developed those antibodies; the presence of them most likely means you were exposed to Covid-19, the illness caused by the virus. Some tests, like the one used by the Mount Sinai Health System in New York, can measure the level of antibodies in your system — your titer.

But in general, most of the tests being made available across the country detect only whether the antibodies are present, said Dr. Jeffrey Jhang, medical director of clinical laboratories and transfusion services for the Mount Sinai Health System. A direct-to-consumer test announced on Tuesday from Quest Diagnostics — more on that below — measures only presence or absence.

Antibodies can take generally anywhere from about a week to 14 days to develop, Dr. Jhang said, and the levels of antibodies vary based on time since exposure and a person’s immune system. This means that a lack of antibodies does not necessarily mean you were not exposed to the virus.

The test is similar to other blood tests you may have had before: A sample of blood is taken from the patient and is then analyzed to determine the presence of antibodies. Most tests will generally return results within a few days, but that may vary, as some tests can return results in a few hours.

But experts generally agree that, based on experiences with other viruses, including SARS, the presence of antibodies most likely does confer some level of protection, though we don’t know to what extent or for how long.

“The difficult thing is we do not have clinical evidence yet of whether the presence of antibodies actually prevents the individual from getting the disease again,” Dr. Jhang said, adding, “I think most people believe that the presence of antibodies in most cases would confer some protection given our experience with other viruses.”

“But we really have to wait to see some evidence of that before we can be confident in being able to say that these antibodies can be protective,” he said.

Remember that antibodies take time to develop, so a lack of antibodies may just mean your body hasn’t had enough time to develop them postinfection.

Widespread antibody testing may also give us a clearer picture of the scope of the disease. Results from a random testing of 3,000 people in New York City recently suggested that as many as one in five residents — or about 2.7 million people — might have encountered the disease without realizing it. When describing the results, Gov. Andrew M. Cuomo said that because the rate of infection might be far higher than initially thought, the death rate of the virus mighty be far lower than we thought.

Once the medical community determines whether antibodies confer immunity — which experts say will take at least six months or so to determine — we’ll have a better sense of who may be less at risk emerging from lockdown.

“Once we understand that the antibodies are protective, then the testing means something,” Dr. Jhang said, as it may help “figure out who can go back to work and be protected and not spread the disease, or when kids can go back to school, teachers going in to teach.”

Further, these three tests confirmed the presence of antibodies in infected people only 90 percent of the time.

Part of the reason for the inaccuracies, Dr. Jhang said, may be what’s called cross-reactivity: This is when a test misidentifies antibodies for a different, but similar, coronavirus.

Florian Krammer of the Icahn School of Medicine at Mount Sinai in New York told The New York Times that false positives weren’t necessarily an issue when determining how widespread the disease is, as a given test’s false-positive rate can be accounted for in estimates. They do, however, matter greatly on an individual level.

“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.

And the World Health Organization, citing ideas for an “immunity passport” or “risk-free certificate” from some countries, last week advised against relying on the tests for policy decisions.

The test is generally intended for people who either have had a positive test for Covid-19 and have recovered; or who think they were exposed to Covid-19 and no longer have symptoms.