Getting an antibody test to see if you had Covid-19 months ago is pointless, according to guidelines issued this week by a major medical company.
Many tests are inaccurate, some look for the wrong antibodies and even the right antibodies disappear, said experts at the Infectious Diseases Society of America, which issued the new guidelines.
Because current tests cannot determine if someone is immune, the company said, they “cannot inform decisions to stop physical distance or reduce the use of personal protective equipment.”
Antibody tests should generally only be used for population tests, not for diagnosing disease in individuals, the panel said. But their guidelines described two situations in which antibody testing could be used when the normal diagnostic tests for the virus – called PCR tests – failed or were likely to fail.
People who felt sick weeks or months ago and now question whether they are immune to Covid-19 “probably should not” get an antibody test, said Dr. Angela M. Caliendo, a testing expert at Brown University’s Alpert Medical School and a member of the society’s expert panel.
Many New Yorkers fell ill in the spring, could not be tested then, and now ask themselves if they had the disease and are immune; the panel concluded that current antibody tests could not answer this question.
“We do not really know if a positive test means you are protected,” said Drs. Caliendo. “If you were sick in March, you probably have no more antibodies; and if you were only mildly ill, you might not have received any antibodies. “
Moreover, she said, “if you live in an area with low prevalence, you have a much higher chance of getting a false-positive test, which means you might think you’re protected, but you are not.”
Despite the lack of antibody tests, recent studies of patients who are fully infected have suggested that they have long-lasting immunity and that it is highly unlikely that they will re-infect.
This may be because white blood cells, known as B and T cells, which are “primary” to recognize and attack the coronavirus, remain in circulation long after antibodies have disappeared. But B and T cells are not analyzed by standard antibody tests.
Other experts doubt society’s recommendation against individual antibody tests.
Michael T. Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy, called diagnostic clinics that charge individuals for antibody tests “snake oil sellers.”
Antibody tests – which analyze blood for proteins produced by the immune system that bind to and neutralize the outside of the virus – are most useful for large-scale studies to see roughly how many people have contracted the disease, the company said.
Even for that purpose, only tests that are positive more than 96 percent of the time and at least 99.5 percent of the time should be used correctly negatively, according to the guidelines.
Very few of the dozens of tests conducted in this country, Europe and Asia, where the panel looked, met these standards, said Drs. Caliendo. None can be done at home or directly in doctors’ offices, and the best are assays known as Elisa or CIA (chemiluminescence immunoassay). Tests with paper strips were less consistently accurate, the panel found.
With two exceptions, antibody tests should not be used to diagnose individual infections, the company said. If a patient has all the symptoms of Covid-19, including X-ray evidence of pneumonia, but is still negative on repeated diagnostic PCR tests for the virus, an antibody test may be helpful.
Ideally, it should be given not immediately after symptoms appear, but three to four weeks later – “the sweet spot,” said Dr. Caliendo.
PCR nasal swab tests can be negative after the virus migrates from the nose and throat to the lungs, explained Dr. Caliendo; but the body may not be able to produce enough antibodies to detect in the blood until more than two weeks have passed. .)
The tests can also be used to diagnose if a doctor suspects a child has multisystem inflammatory syndrome, a rare but serious complication of Covid-19 in children. Because it is not known how long after the initial infection this inflammation begins, doctors would need to do both a PCR test and an antibody test, the guidelines said.
The Centers for Disease Control and Prevention has also issued guidelines for the use of antibody tests; they were last updated on August 1. The guidance is based on validations of test kits by the National Cancer Institute, while the Society for Infectious Diseases monitors test results from around the world, said Dr. Kimberly E. Hanson, a test expert at the University of Utah Hospital who was also on the expert panel .
The panel found it difficult to decide which tests were worth recommending.
The Food and Drug Administration’s decision in April to approve tests without even checking its safety and efficacy data “proved to be a problem,” said Drs. Hanson. “The market was flooded with poor performance testing.”
After harsh criticism from expert testers and Congress, the FDA reversed that decision in May and gave test makers 10 days to prove that their tests were accurate or were banned from selling it.
On Wednesday, the Department of Health and Human Services, quoting an executive order from President Trump, said laboratories developing their own tests for the virus should not pass the FDA review. The order does not apply to the commercial tests that the panel analyzed.
The tests that are still on the market right now are better, but still not good enough, experts said.
The panel examined 9,500 papers describing test validation and found only 47 worth considering. “It was a mishandling of games,” said Drs. Hanson.
Dr. Caliendo said: “We naively thought this would be easy. There are not many high quality results on these tests.”
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Frequently Asked Questions
Updated August 17, 2020
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Why does standing six feet apart help others?
- The coronavirus spreads primarily by drips from your mouth and nose, especially when you cough or sneeze. The CDC, one of the organizations using this measure, bases its six-foot recommendation on the idea that most large droplets that expel people when they cough or sneeze fall within six feet to the ground. But six feet has never been a magic number that guarantees complete protection. Noses, for example, can launch drops much farther than six feet, according to a recent study. It’s a rule of thumb: you need to be the safest to stand six feet apart, especially when it’s windy. But always wear a mask, even if you think you are far enough apart.
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I have antibodies. Am I immune now?
- At the moment, that probably seems like at least several months. There have been horrific accounts of people suffering from what appears to be a second bolt of Covid-19. But experts say these patients may have a prolonged course of infection, with the virus taking a slow toll weeks to months after initial exposure. People infected with the coronavirus typically produce immune molecules called antibodies, which are protective proteins made in response to an infection. These antibodies may only last two to three months in the body, which may seem worrying, but that is perfectly normal after an acute infection progresses, said Dr. Michael Mina, an immunologist at Harvard University. It could possibly get the coronavirus back up, but it is highly unlikely that it would be possible in a short window of time from initial infection or the second time safe.
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I am a small business owner. Can I get relief?
- The incentive bills introduced in March provide assistance to the millions of American small businesses. Those eligible for support are non-profit companies and organizations with less than 500 workers, including sole proprietorships, independent contractors and freelancers. Some larger companies in some sectors are also eligible. The assistance provided, which is managed by the Small Business Administration, includes the Paycheck Protection Program and the Economic Damage Disaster Program. But many people have not seen payments yet. Even those who have received help are confused: the rules are draconian, and some are stuck on money they do not know how to use. Many small business owners get less than they expected or heard nothing at all.
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What are my rights if I am worried about returning to work?
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What will school look like in September?
- It is unlikely that many schools will return to a regular schedule this fall, requiring the grinding of online learning, easy childcare and stunted workdays to continue. The two largest public school districts of California – Los Angeles and San Diego – said on July 13 that instruction will be remote only in the fall, citing concerns that growing coronavirus infections in their areas pose too great a risk to students and teachers. Together, the two districts enroll about 825,000 students. They are the largest in the country to date to abandon plans for even a partial physical return to classes when they reopen in August. For other districts, the solution will not be an all-or-nothing approach. Many systems, including the nation’s largest, New York City, are creating hybrid plans that include some days attending classrooms and other days online. There is no national policy yet, so check regularly with your municipal school system to see what is happening in your municipality.
Even in those 47 papers, the same tests performed better or worse in different studies, they said.
Tests that do not detect so-called IgG antibodies, which begin to appear about two weeks after infection, are not helpful, said Dr. Hanson.
There are two other types: IgM antibodies appear later in SARS-CoV-2 infections than they do with other viruses, are less specific than IgG antibodies, and levels fluctuate more, she said.
IgA antibody tests do not effectively distinguish SARS-CoV-2 from other coronaviruses that cause common colds.
Even for population surveys, tests need to be highly accurate, experts said. In populations with only a few people infected, even an uncuracy rate of 1 percent wide margins of error can be introduced.
Antibody tests are different from antigen tests, which look for a protein on the surface of the virus. They are not considered very accurate, but give results in 15 minutes. Because laboratory backups and lack of reagent testing have delayed more accurate PCR testing by as much as two weeks in some cities, many institutions have switched to using antigen tests.
Dr. Hanson said the company knew there was “a lot of interest” in antigen testing.
“But we could not even get our hands on it to evaluate,” she said. “There are too many manufacturers with emergency use authorization, and the demand is high.”
A positive result on an antigen test is generally considered accurate, she said, but a negative means the patient needs to be re-tested with a PCR test.
Dr. Caliendo said her hospital in Rhode Island did not use antigen tests at all, and local schools used them to test only children who were clearly ill.
“We do things that are less than ideal because we can not get the materials we need,” she added.
Society is looking at the new detection tests, which appear to be as sensitive as nasal swab tests, said Drs. Hanson, “but there is not much data on how long the virus is in saliva.”
Also, she said, “saliva is bubbly and difficult to pipette.” As a result, labs report 3 percent of tests invalid, compared to 1 percent for swab tests, “which is frustrating.”
On the other hand, she said, swabbing can hurt a patient, putting the person giving the tests at risk. Spitting does not work, said Dr. Hanson, “so you do not need the same level of PPE – that’s important, because here in Utah it’s 100 degrees in the test tents.”
