You have heard about them everywhere since the beginning of the health crisis; antibody tests that can tell you if you have had an undiagnosed COVID-19 case in the past and if you may be immune to the disease. Apply that concept to an entire community and you can determine the actual rate of COVID-19 penetration. Studies from Spain and comments from the CDC have said in recent months that the actual infection rate could be close to 10 times higher than what has been reported. Antibody tests are also needed in vaccine studies, to prove if the immune system has started increasing armies of antibodies that can block the virus when the real thing comes.
But recent studies have shown that antibodies decrease over time and they may disappear from the bloodstream within three months. COVID-19 immunity is not lost when that happens, because hordes of T cells maintain a memory of the infection and can help kill the pathogen during reinfection. Studies have also shown that current antibody tests could give false-negative results in patients who had COVID-19 in the past.
All of this brings us to new guidelines that say that antibody tests are pretty useless to individuals, although there are cases where they can be useful.
The Infectious Disease Society of America (IDSA) has published a comprehensive paper on COVID-19 serological testing.
The organization found that testing for antibodies was not good enough to “determine immunity or risk of reinfection.” The tests “may not inform decisions to stop physical distance or reduce the use of personal protective equipment.” That’s a more complex way of saying you’re lucky when you’re trying to determine if that cold or flu-like illness back in February of March was really COVID-19. Another implication is that the so-called immunity passports that people keep talking about are completely out of the question. If current tests of antibodies are not good enough to prove that one had COVID-19, they can not be used for issuing any kind of documentation that would say so much. And antibody testing is the only way hospitals can now directly check the immunity of COVID-19. Testing T cells and B cells is much more complicated and not so widely available.
The IDSA panel would like to think that antibody tests have some limited use, including one that contradicts the opposite point somewhat. The society says that antibody tests can be used in epidemiological studies that look at herd immunity in communities. But if antibody tests cannot detect older infections, as we have just established, those herd immunity studies may not paint an accurate picture of a city as COVID-19 immunity of a country. Tests could pick up patients who were recently infected but others may be missing, which is exactly what a recent study revealed. Therefore, such research would always provide a lower percentage of immunity in a community.
IDSA says there are two clinical scenarios in which antibody testing has potential utility. One includes the “evaluation of individual patients with a high clinical suspicion for COVID-19” who remain negative tests, or who do not have access to PCR tests. But even this kind of use may not always yield accurate results. The paper notes that there may be problems with antibody testing that may hinder this work:
In general, IgM tests have a lower susceptibility to detection of past infection than IgG or total antibody tests. Assays designed to detect and differentiate IgM and IgG in combination, in which the detection of or IgM as IgG is used to define a positive test result and IgA tests have lower specificity to detect past infection compared to IgG alone as total antibody tests.
IgM, IgG and IgA are types of Y-shaped immunoglobulins, as antibodies. IgM antibodies appear later in COVID-19 than in other viruses, IgG antibodies are detected two weeks after infection, and IgA antibodies are not as good at killing SARS-CoV-2 as other human coronaviruses.
The paper also notes that the sensitivity and specificity of IgG and total antibody counts are optimal three to four weeks after the onset of symptoms. By the time antibody tests are optimal, people suspected of having an infection may have already wiped out the virus.
There is a third instance in which serological tests may be useful and that is “for assessments of suspected multisystem inflammatory syndrome in children.” That is the MIS-C syndrome that some children experience, which can be fatal.
Not all hope has been lost for using antibody tests to map COVID-19 immunity. But tests must have a high clinical sensitivity and specificity (more than 99.5 percent) “to be of value.” Manufacturers of antibody tests should come up with improved test kits that would have very low false-negative and false-positive results. At present, it is believed that many antibody tests are not accurate. Even for research on herd immunity, the ISDA panel advises that tests should be correctly positive 96 percent of the time and correctly negative at least 99.5 percent of the time. The majority of test kits that studied the panel did not meet these standards, Drs. Angela Caliendo to The New York Times. None of them are available for testing at home. The best types of antibody tests are Elisa or CIA (chemiluminescence immunoassay), so this is the type of serological test you should look for.
One of the problems with antibody testing in America is the Food and Drug Administration’s decision to make them good without checking safety and effectiveness data. That decision was reversed in May, with the FDA giving test makers 10 days to prove their assays were accurate. But on Wednesday, the Department of Health and Human Services said that labs that perform their tests no longer have to go through an FDA evaluation, per an executive order from Trump. The order does not apply to commercial tests, but the tests you can buy right now are not good enough yet.
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