Antibody tests can give you false report of COVID-19 immunity, CDC warns
People are depending on widely available antibody tests to determine if they were once infected with the novel coronavirus, but health officials now say that may not be a good idea.
The Centers for Disease Control and Prevention updated its coronavirus testing guidelines Monday and said that antibody tests, also known as serology tests, can get it wrong about half of the time, meaning some with a positive result will roam freely but ignorantly, putting themselves and others at risk of COVID-19 infection.
Although the CDC recommends against antibody testing to diagnose past infection, the agency still touts the test’s ability to determine which communities are immune or protected from the virus and even those who may qualify to donate blood as treatment for people seriously affected by COVID-19.
“Serologic test results should not be used to make decisions about grouping persons residing in or being admitted to congregate settings, such as schools, dormitories, or correctional facilities,” the CDC said. The tests should also “not be used to make decisions about returning persons to the workplace.”
“Until more information is available … testing for antibodies is not recommended,” the agency added.
Antibodies are proteins the immune system produces to help stop foreign invaders such as viruses and bacteria. Their presence in blood — usually within one week of being sick — indicates a person was once infected with a certain pathogen.
In contrast, nose and throat swabs like those being done today are used to test if someone currently has the disease in question.
But SARS-CoV-2 is complicated, according to the CDC, because its particular antibodies called IgM and IgG appear later in the process, two to three weeks after infection.
At the same time, how long these antibodies remain detectable and whether they guarantee protection from reinfection of the coronavirus remains unclear, the CDC said.
But one thing is certain: the accuracy of antibody tests depends on how widespread a COVID-19 outbreak is in a given population.
For example, in food processing plants and nursing homes, where many people are in the same enclosed space, the “positive predictive value” of the antibody test increases.
This means it’s more likely that those who test positive for having antibodies in their blood actually have them.
On the other hand, for environments with fewer coronavirus cases and lower chances of community spread, “the positive predictive value drops because there are more false-positive results,” the CDC said.
“In most of the country, including areas that have been heavily impacted, the prevalence of SARS-CoV-2 antibody is expected to be low, ranging from <5% to 25%, so that testing at this point might result in relatively more false positive results,” the agency said.
In simpler terms: not enough Americans have been infected with the coronavirus for the antibody tests to prove helpful in understanding how many people were actually infected nationwide, and how many might be immune today.
The CDC also said that some antibody tests may confuse SARS-CoV-2 with other coronaviruses, “such as those that cause the common cold.”
What’s more, some people may not develop antibodies at all, or levels of the protein could drop over time until they are no longer detectable in the blood, the CDC added.
“Thus, serologic test results do not indicate with certainty the presence or absence of current or previous infection with SARS-CoV-2.”
The U.S. Food and Drug Administration approved an emergency use authorization (EUA) for the antibody test for use by medical professionals, and requires those “commercially marketed” to receive an EUA before use.
But some who receive positive results still think they can’t get infected with COVID-19 a second time.
“I think people just want this to go away and want to resume their normal lives,” Kelly Wroblewski, director of infectious disease for the Association of Public Health Laboratories, told NPR. “But my fear is [antibody tests are] going to be used as this sort of golden ticket to demonstrate immunity — when we just don’t know if that’s the case.”
The CDC instead recommends health care workers use viral — nucleic acid or antigen —tests to diagnose infection.
Nucleic acid tests involve a nose swab and can detect the presence of coronavirus genetic material in patient samples, according to the American Society for Microbiology.
Antigen tests also require nose swabs. They can see if respiratory samples have proteins usually found on or within the coronavirus, according to the FDA that approved an EUA for one such test in early May.
This story was originally published May 27, 2020 at 6:02 PM with the headline "Antibody tests can give you false report of COVID-19 immunity, CDC warns."
