Antibody tests for coronavirus may go unnoticed. News

Dozens of blood tests are quickly coming to market to identify people who have been exposed to the coronavirus by looking for antibodies to it.

The Food and Drug Administration does not regulate these types of tests, but even those that meet the informal government standard can produce many false answers and provide false guarantees. The imperfect results could be a major disappointment for people looking for these tests to help them return to something akin to a normal life.

First of all, it is not clear whether someone who has antibodies against the coronavirus in their blood is really immune. Your body makes these antibodies within about a week of infection.

In many other diseases, people have a period of immunity after being exposed to a microbe and recovering from the disease. But that has not yet been proven with the coronavirus.

Another problem is that the test results are incorrect much more frequently than might be expected. While the tests can honestly say they are more than 90% accurate, in practical use they can often perform well below that level.

Deborah Vander Gaast of Tipton, Iowa, I would love to hear the results of an antibody test.

“What if we are already immune and we just don’t know it and we don’t have to be afraid?” she asks. VanderGaast runs a day care center for children with developmental and behavioral disabilities. They look a lot like young children everywhere.

“We laughed at disinfecting everything because it only got reinfected only two seconds later,” he says with a smile.

VanderGaast eagerly awaits the launch of a blood test that could determine if she and her staff have antibodies to the coronavirus. They are now hypervigilant and remain at home for 14 days if they were even potentially exposed to the coronavirus. They wouldn’t have to do that if they knew they were immune, she says.

VanderGaast says the test is not available in his county right now. But it is starting to take off nationally.

Dr. Jeremy Gabrysch runs a mobile medical service in Austin, Texas. He obtained a supply of antibody tests from a major Chinese manufacturer and says he has tested several hundred people in the past few days.

“We offer the test for people who may have suspected they may have had a coronavirus in February or March when testing with the nasal swab [and PCR diagnostic test] It was very limited, “he says. The fee: $ 49 per test.

Gabrysch says it only tests people when it has other evidence that they might have been exposed. “If they had an illness that looks like it could have been coronavirus and they had a positive antibody test, then it is very likely to be a true positive, that they actually had COVID-19,” he says.

The test you’re using, produced by Guangzhou Wondfo Biotech in China, has a specificity of 99%, which means it only falsely says that a blood sample contains antibodies to the coronavirus when it is not 1% of the time. But despite that impressive statistic, a test like that is not 99% correct, and in fact in some circumstances could be much worse.

This is due to this counterintuitive fact: the validity of a test depends not only on technology, but on how common the disease is in the sample population.

“It’s kind of weird,” admits Dr. H. Gilbert Welch, a scientist at Brigham and Women’s Hospital in Boston who studies problems related to testing and screening. “An antibody test is much more likely to be incorrect in a population with very little exposure to COVID.”

This is the result of statistics, rather than the technology of any given test.

Here is a simple way to see it. Let’s say you are running a test that gives five false positive results in a group of 100 people. That doesn’t sound too bad. But consider this. If 5% of those 100 people were actually infected with the coronavirus, you should get five correct test results (true positives), along with the five false positive results.

While the manufacturer can rightly claim that the test is 95% specific, in this population “the test will be wrong half the time,” says Welch. “Half of the people will be falsely reassured.”

And the test results can be considerably worse.

The Food and Drug Administration does not regulate these tests, but White House Coronavirus Task Force Coordinator Dr. Deborah Birx has said she expects manufacturers to achieve a 90% specificity standard (and 90 % sensitivity, another measure of test performance that is less important (in this context).

This is what would happen if you were to use a test with 90% specificity in a population where only 1% of people have coronavirus. No one knows for sure, but that could be the situation in many parts of the country.

In that case, over 90% of the positive results would be false positives and falsely reassuring. (You can run your own examples on the calculator on this page.)

The tests now used vary widely in their specificity.

Laboratory giant Becton Dickinson says his coronavirus antibody test has a specificity of 91%. Emory University says that its test has a specificity of 95%. Stanford University says its test showed 100% specificity, at least in a preliminary study with 100 samples. The Centers for Disease Control and Prevention has been developing an antibody test since the early days of the epidemic, but has not yet published results on its performance.

One way to limit the problem of false positive results is to focus on populations where the disease is most common; in other words, in situations where true positives are much more frequent than false positives. That is likely to be the case in hospitals that have cared for patients with coronavirus.

“It would be wonderful for healthcare workers to know their immune status, just to give them peace of mind,” says Dr. Jordan Laser, a pathologist at Northwell Health in Long Island, New York.

Still, Laser says it would still be a mistake to trust these results.

“Definitely don’t use these tests to change your practices in terms of wearing personal protective equipment,” he would advise his colleagues. “You definitely don’t feel comfortable doing your job caring for patients with COVID. It really would be more of a psychological benefit. “

Jeremy Gabrysch, the Austin physician, is also using these tests to satisfy his patients’ curiosity, not to provide practical guidance on whether it’s safe for people to assume they’re immune.

“We don’t recommend that people interpret the result that way, and we discourage that,” he says. “It is not a card to get out of jail.”

Due to these significant limitations, antibody testing is not the imagined passport that would allow people to return to their normal activities and ignore the coronavirus.

But high-quality antibody tests will still be valuable in cases where individual false-positive results matter less. That would be the case of whole population surveys, where errors can be eliminated.

In these studies, antibody tests are used to answer critical questions about where the coronavirus is and how prevalent it is. That information can help officials plan how to allow normal life to resume, but in all populations as a whole, rather than one person at a time.

You can contact NPR Science correspondent Richard Harris at [email protected].

A conversation with California biologist Andrew Cohen inspired this line of research.

Copyright 2020 NPR. To see more, visit https://www.npr.org.

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