How useful are antibody tests?



ELLSWORTH — It’s a question most of us would likely have never imagined asking a few months ago, but one that’s been on the minds of many in recent weeks: If a test is available to look for antibodies against the virus that causes COVID-19, should I get one?

You could, but at the moment there’s a fairly decent chance the results could be a.) inaccurate and b.) even if they are accurate, not necessarily that useful.

“Antibody testing is not designed to look for a current, or active infection,” Maine Center for Disease Control and Prevention Director Dr. Nirav Shah explained at a briefing on Monday. “Rather, it’s a little bit like looking at footprints on the beach. It tells you that perhaps somebody walked by but you’re not sure when, you may not be sure who and you may not be sure whether they’re coming back or not.”

Testing of all kinds has become the holy grail of the coronavirus: it’s been hard to come by and experts say a lot more of it is necessary to get the country back to any semblance of normal.

There are currently three different types of tests on the market, said Dr. Mike Murnik, a family physician and senior physician executive with Northern Light Blue Hill Hospital. That includes antibody tests (also known as antibody blood tests or serologic tests), rapid antigen tests and viral tests (also known as diagnostic, molecular or polymerase chain reaction, PCR, tests). The first looks for evidence that a person may have been infected with a virus at some point in the past, while the latter two look for active infections.

“The only one that we’ve seen a lot of so far, at least locally,” said Murnik, “is the viral test. Molecular, PCR, all the same thing. These are the nasopharyngeal swabs.”

The viral test amplifies the genetic material of the virus, said Murnik, and tells health-care workers whether the material is present or not.

Depending on how far the swab has to be sent for processing, those tests can be turned around fairly quickly, although most take at least a day, if not several. Abbott Laboratories has developed one that can give results in as little as 15 minutes, although there has been one study reported on by National Public Radio that suggested the test had a false negative rate of roughly 15 percent.

“A positive result is something that we can have faith in,” said Shah at a briefing in April of the Abbott rapid-response tests. “A positive result on the Abbott machine suggests that the person does in fact have COVID. But a negative result is a bit more in question.”

On the whole, however, the diagnostic tests are fairly reliable, said Murnik.

“There seems to be a false negative rate,” he said, “and we don’t quite know what that is. It seems to be swab-related; if the swab doesn’t go to the right place, it could come back negative.”

“Nothing’s 100 percent,” Murnik added. “Nothing’s ever 100 percent.”

Antigen testing, which is also used to diagnose an active infection, uses a different method than looking at the genetic material of the virus in a mucus-filled swab; instead, it looks for the antigens.

That’s “the protein on the surface of the virus that generally makes antibodies reacting to the virus,” said Murnik. “This would be just like the rapid strep test that we do in offices or rapid flu tests,” which return results very quickly, within 10 or 15 minutes. But Murnik said he hasn’t yet seen any of those test kits in use in the area.

Then there are the antibody tests that have started to come online in recent weeks. But in part because the Food and Drug Administration (FDA) has waived many of the normal rules in an effort to get tests on the market, said Murnik, a lot remains to be seen about how accurate and how useful they are.

“There’s some real quality concerns about what’s out there,” said Murnik. Some appear to be no more accurate than flipping a coin, Shah said on Monday, and “It’s really difficult to know whether the test you might be getting is one of the really good ones.”

There are also some other problems, not the least of which is that SARS-CoV-2, the virus that causes COVID-19, is one of a large family of coronaviruses.

“And it’s got a lot of cousins, and those cousins look really similar,” said Shah. “Some of the antibody tests that are on the market right now can’t really tell the difference between the virus that causes COVID-19 or the virus that causes the common cold.”

It can also take up to six weeks to develop antibodies, said Murnik, meaning if you’re tested for them too soon after you’ve been infected they might not show up, even if you actually were infected.

Beyond all of that, even if a patient were to get one of the more accurate antibody tests, said Murnik, and the results came back positive for the virus that causes COVID-19, scientists are yet really sure how to interpret those results.

“We don’t know if those antibodies mean that you’re immune,” said Murnik, or how long that immunity might last. You might be able to get the virus again in a few months, like the common cold, or those antibodies might provide protection for life, like in the case of chickenpox or polio.

The value of an antibody test is also limited, as Tara Smith, a professor of epidemiology at the Kent State University College of Public Health, explained to Scientific American, by the reality that tests are more likely to give false positives (you are negative and test positive) in a community with a lower prevalence of disease. That could be problematic.

“There is the potential for individuals to be misled regarding their [antibody] status,” Smith said. “If they are false positive, they may believe they are immune when they are not and may relax protective measures.”

On the contrary, explains Scientific American, “In populations with a higher prevalence of a disease or past exposure to it, true positives (individuals who test positive and have antibodies to the illness from a prior infection) and false negatives (those who test negative but actually have antibodies) are more common.”

Without more widespread diagnostic testing, which has been held up by a shortage of testing kits, swabs and other materials needed to run them, said Murnik, “We don’t know the true incidence of disease. We have estimates, but they’re all skewed because we’re only testing the sickest people.”

“We don’t really have a good measure of what’s going on in the community. We don’t know what the true predictive value is,” he said, “which really limits the value” of what researchers and doctors can tell a patient from an antibody test.

If the opportunity to get an antibody test comes up (some firms in southern Maine have begun offering the tests), Shah advises Mainers to ask three questions of their provider: “Which machine are you using — Is it one of the good ones? If I get the result that’s positive, how will you interpret it?” And “What do we know right now about the science around how long that protection might be good for?”

One of the best ways to protect yourself against the virus, said Murnik, remains physical distancing.

“It’s really physical distancing that makes the difference,” said Murnik. “It does work. Masks definitely improve your odds.”

“I think we need to pay attention to our public health principles as we work on reopening,” said Murnik, while science catches up. “We need to work together in everyone’s best interest to figure out how to dance with this thing.”

Kate Cough

Kate Cough

Kate covers the city of Ellsworth, including the Ellsworth School Department and the city police beat, as well as the towns of Amherst, Aurora, Eastbrook, Great Pond, Mariaville, Osborn, Otis and Waltham. She lives in Bar Harbor and welcomes story tips and ideas. She can be reached at [email protected]
Kate Cough

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