The most frequent question we have heard over the past several weeks or so has been, “can I get an antibody test for COVID?”
The short answer is that we can send the blood test, but at this point, we advise against doing so. A recent Wall Street Journal article was headlined, “Among New Coronavirus Antibody Tests, Limited Value and Many Unknowns” and that’s a pretty good summary.
Antibodies are proteins made by our immune system to fight infection. There are several types and we primarily use tests for IgM and IgG in clinical practice. IgM is produced early in an acute infection, IgG follows days or weeks later.
There are two questions we’d like to be able to answer with an antibody blood test.
Have I had COVID-19?
Am I immune to COVID-19 based on having been exposed?
At this point, we can answer the first question fairly confidently, but not the second.
Any antibody testing performed in a physician’s office is likely worthless. This week, the FDA rescinded emergency use authorizations for antibody testing and will now require more rigorous study before they are approved. "Flexibility never meant we would allow fraud. We, unfortunately, see unscrupulous actors marketing fraudulent test kits and using the pandemic as an opportunity to take advantage of Americans' anxiety."
Antibody testing is available and tests done by high-quality reference laboratories now appear to be specific for SARS-CoV-2 (there was previously a concern that they might detect the presence of other coronaviruses). It appears that Quest is using two tests (EUROIMMUN and Abbott Architect) depending on supply availability. They have a sensitivity (the ability to detect antibodies when present) of 90 and 100% and specificity (meaning that when the test is positive, it is a real finding and not false-positive) of 100 and 99.6% respectively. So, while the EUROIMMUN test may be negative in 10% of patients who have been infected, neither test has a significant false-positive rate and this is reassuring.
The problem is that we do not know whether having IgG antibodies means that you cannot get the disease again or spread it to others. This is a very important issue and the subject of many studies. If antibodies are protective, a vaccine program is more likely to succeed. And if we can use the test to identify people previously infected (again if the antibody prevents repeat infection), that would be very valuable as well. Of course, the next step would be to determine the duration of protection with a positive antibody test.
At present, the utility of IgG testing in an individual is limited to possibly identifying people who might be able to donate plasma for use in experimental treatment of severe COVID-19 disease. Of course, public health officials are testing large groups of people to determine how much the disease has spread in various parts of the country.
Hopefully, IgG will prove to be protective. But hope is not a scientific discipline.
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