Antibody tests give clues on COVID exposure

This scanning electron microscope image shows SARS-CoV-2 (round gold objects) emerging from the surface of cells cultured in the lab. SARS-CoV-2, also known as 2019-nCoV, is the virus that causes COVID-19. The virus shown was isolated from a patient in the U.S. Credit: NIAID-RML

KANSAS CITY, Mo. — Glenn Crocker of Overland Park was looking for answers.

Is it possible that he already had COVID-19 and just didn’t know it? Maybe he is the one, in some infectious but asymptomatic way, who had passed the coronavirus on to his 16-year-old daughter, Elizabeth, and forced her into quarantine in her room.

It was April, about a week after Elizabeth’s 16th birthday. Then a sophomore at Blue Valley North High School, she had developed the hallmark dry cough, a slight fever. The Crocker family, Glenn and his wife, Tepring, their eldest child, Ben, and Elizabeth, had all been vigilant, wearing masks and social distancing.

But on occasions, Glenn and Tepring would go to a local coffee shop. Crocker wondered if he caught it and unwittingly passed it on.

“It’s my fault,” Crocker said he thought. “I infected her.”

To confirm or ease his worst fears, Crocker took a COVID-19 antibody test, now being offered — typically for $130 to $200 before possible insurance coverage — in the Kansas City area at private laboratories such as NextCare Urgent Care or Quest Diagnostic.

They’re blood tests. Individuals can make appointments online or through a physician.

Unlike the nasal swab tests, they do not determine if an individual has an active coronavirus infection. They determine if one has already been exposed to the virus and thus built up antibodies. Antibodies take one to three weeks to develop after infection.

The Centers for Disease Control and Prevention estimates that as many as 35% of individuals with COVID-19 may not show symptoms. Or the symptoms are milder, like a cold or flu. As such, it’s unknown how many people already, without their knowledge, had the illness, got over it, and have some level of immunity.

Health experts have begun using antibody tests to determine the prevalence, or rate, of COVID-19 in certain places.

The Kansas City Fire Department, working with researchers at the University of Miami and the University of Missouri-Kansas City, hoped this week to begin offering antibody testing to its 1,300 firefighters and paramedics. Subsequent tests would be taken at 30, 90 and 180 days afterward to monitor any growth in the illness.

“We’ve had a lot of people who have thought they had COVID already,” said Chief Donna Maize. “We can gauge how it is hitting our department.”

The department already offers COVID-19 testing for personnel who think they might have active infections. She said about 190 employees have been quarantined because they had the disease or suspected they did.

Antibody testing adds data about how much the illness may have spread.

In Florida, for example, the state Department of Health released data showing that about 4.4% of some 123,500 people tested were found to have antibodies to the illness. In Miami-Dade County, 6% of 1,400 people tested were positive. Half showed no symptoms.

The Kansas Department of Health and Environment also plans to do broad antibody testing, although a timeline hasn’t been announced.

Crocker got his antibody test result in about two days. Negative: no antibodies.

“I feel like, great, I’m not how she caught it,” Crocker said of his daughter. “And if she had it, I didn’t catch it.”

Crocker said “if” because by the time they got Elizabeth in for the regular nasal swab COVID-19 test, her symptoms had passed and she came up negative. The next step might be to get her an antibody test.

Physicians, however, caution people not to read too much into antibody results.

The most common question is whether testing positive for antibodies means you can roam the world with impunity, safe from getting COVID-19 again.

“I’m going to answer that with a solid maybe,” said physician Jed Gorlin, medical director of the Community Blood Center. “We don’t know.”

Having antibodies to the coronavirus means that you have been exposed to it enough to trigger an immune response. But not all immune responses are the same. If you have antibodies to measles, it means you’re immune to measles. But having antibodies to herpes doesn’t mean you can’t get it.

It is possible, even likely, some doctors say, that COVID-19 antibodies do offer some protection against getting reinfected. But no one knows how great that protection is, or how long it might last.

“Is it a year? Is it a month?” said Melvin McFarlin, physician with NextCare Urgent Care. “We don’t know how much immunity you’re actually getting.”

Even if you test positive for antibodies, physicians still recommend that patients take the recommended precautions: continue to social distance, disinfect, wear masks.

Using an Emergency Use Authorization, the federal Food and Drug Administration on April 28 allowed for a number of antibody tests. Although highly sensitive, they are not 100% accurate.

There is a small possibility of false positives (tests say there are antibodies when there are not) and false negatives (says there are no antibodies, when there are). Positive tests are more likely to be accurate in areas such as New York City or New Orleans, where there is already a high prevalence of COVID-19.

Companies such as nursing homes and food processing plants have been using antibody tests to screen their employees. At NextCare in the Kansas City area, a theater company had its actors tested.

In its list of recommendation, the CDC says:

— Antibody tests should not be used to determine if someone can return to work

— Antibody tests should not be used to group people together in settings such as schools, dormitories and correctional facilities.

— People who wear personal protective equipment at work should continue to wear it even if they test positive for antibodies.

“People should ask themselves, ‘Why do I want this test?’” Gorlin said.

For studies tracking the illness, it’s valuable. For people who want to check to see if the horrible cough and fever they had months ago might have been COVID-19, it might be valuable.

“If they want this test so they can relax their guard,” he said, “they may be acting prematurely.”

Tribune Wire

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(1) comment

Holmes -- the real one

The antibody test does not pick up every case where antibodies are present.

But the ones it does pick up tend to be true cases with antibodies (not false positives.

There is a reason why structuring a test this way is useful -- but it does mean that you might have antibodies but there is the possibility that they won't be recognized by the test.

Another consideration is the question of just how long the antibody response lasts. There is growing evidence that it may not be a prolonged response. That, of course, has implications for the use of a vaccine. Booster doses may be required.

Accuracy and Specificity have differing roles as far as testing is concerned.

Here is a discussion of that particular subject:

Key Role of Specificity in COVID-19 Antibody Test Accuracy

https://diagnostics.roche.com/us/en/roche-blog/key-role-of-specificity-in-covid-19-antibody-test-accuracy.html

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