Doctors say employers should not use COVID-19 antibody tests to decide whether employees are safe to return to work, yet such testing is being promoted by lab companies and hospitals to businesses through “back to work” programs.

The idea is tantalizing: If scientists knew a COVID-19 infection caused the body to produce antibodies that reliably protect against re-infection, determining who’s safe to return to work could be as simple as a well-designed blood test.

Yet the American Medical Association, the Centers for Disease Control and Prevention and the Minnesota Department of Health each say the evidence backing test accuracy and protectiveness from antibodies is not yet strong enough. Even the lab companies and hospitals admit they can’t offer “immunity certificates” to people who have the antibodies today.

“We don’t know what level of antibodies makes someone immune to COVID-19. All we can tell is that the person has been exposed,” said Dr. Susan Bailey, a Texas allergist and immunologist who is president of the American Medical Association. “We don’t necessarily know what that immune response means, in terms of someone’s ability to go back to work or to school.”

National lab companies like Quest Diagnostics and LabCorp, as well as Minnesota health care providers such as HealthPartners in Bloomington, North Memorial Health in Robbinsdale and Mayo Clinic in Rochester, are all offering antibody tests as part of a suite of “return to work” and “stay at work” offerings.

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Those organizations say any decision to use antibody testing in a workforce is based on individual factors, such as which industry is involved, the proximity of workers to other people, and the overall exposure level to COVID-19 in a community.

Antibody tests are typically used together with diagnostic testing to give a fuller picture of a person’s infection status. Specimens can be drawn at work, at a clinic or elsewhere.

“We would consult with our lab experts and our occupational medicine experts and ensure that we’re using the best evidence … to make the recommendations that support what they’re needing to accomplish for the business,” said Dr. Kevin Ronneberg, associate medical director overseeing health initiatives like the “Back to Business” testing program at HealthPartners.

HealthPartners says all samples that test positive for COVID-19 antibodies are run through a second instrument from a different company, per federal recommendations, leading to what the system said is a 99.9% positive predictive value for the IgG antibody.

But testing accuracy can’t make up for the fact that researchers don’t know yet whether antibodies to the SARS-CoV-2 virus are totally protective, how long they last, or why some people don’t produce them after having COVID-19.

The coronavirus that causes COVID-19 was discovered late last year in China. Although antibodies to past respiratory syndromes like SARS and MERS last a year or more, a study of 37 asymptomatic COVID-19 patients in China reported last month that antibodies to the new coronavirus declined within three months of initial infection, study authors wrote in Nature Medicine.

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A second study, not yet peer reviewed, examined antibodies in 5,300 Chinese health care workers and COVID-19 patients and concluded last month that “people are unlikely to produce long-lasting protective antibodies against this virus.”

Antibody blood tests are one of the two kinds of COVID-19 screens people will encounter at work. The other kind is “molecular” PCR testing, using nasal or throat swabs. PCR tests are the only tools that can diagnose active cases and have prompted long lines in cities with outbreaks.

On Sunday, PCR testing confirmed 715 new cases of COVID-19 in Minnesota, bringing the state’s tally to 42,281 since March 5. The Minnesota Department of Health also reported three new deaths from COVID-19, bringing the state’s tally to 1,502. Nationally, nearly 135,000 have died.

But the death toll from COVID-19 has not stopped even high-contagion businesses such as meatpacking plants, hair salons and bars from reopening.

And last week, 64% of Minnesota parents in an informal online survey with 130,000 responses said they would feel comfortable sending their students into school buildings in the fall.

Dr. William Morice, an immunologist and blood disorder doctor who is president of Mayo Clinic Laboratories, said antibody testing can be useful in workplace settings, despite the drawbacks and caveats.

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“It gives you a sense for how much of your workforce has been exposed to the virus, and we know there is some level of protective immunity from that — we don’t know how much,” Morice said.

The first company to sign up for Mayo Clinic Laboratories’ comprehensive antibody and PCR testing services is Delta Air Lines, which is testing its entire workforce.

North Memorial Health started working directly with smaller and midsize companies last month. A spokeswoman said employers make their own decisions about what screening to do and what value it offers. North Memorial recommends employers in its testing program screen their employees every 14 days.

HealthPartners’ “Back to Business” program has signed up about 20 employers as clients from among 200 initial inquiries since mid-May. Clients include retail and manufacturing businesses and public entities like cities and school districts. The system’s laboratory experts and occupational-medicine doctors are consulted to ensure the best evidence is used.

“As you know, that is changing frequently,” Ronnenberg said last week, adding that test usage “will be determined based on business needs and the best available medical evidence.”

Though employees are often provided the results of their work-sponsored antibody testing, AMA President Bailey said people should talk to their doctors about understanding the results.

Testing “needs to be done taking that person’s individual case into account and looking at the total picture of their health,” Bailey said. “And I would caution against the validity of any kind of immunity certificate that was given at this point in time.”

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