Coronavirus antibody tests aren’t accurate!

Doctors, medical administrators and consumers are falling for misleading marketing of Covid-19 antibody tests. And the FDA has exerted almost no oversight over a hodgepodge of different tests that have come on the market since April, promoted with a blitz of direct emails to clinics and hospitals that tout the tests’ ability to detect past infections of Sars-CoV2.
The FDA made the opposite mistake with the diagnostic tests meant to flag active infections. Those rolled out too slowly, when even imperfect tests would have saved lives at that early stage. There’s no reason to rush out antibody tests since there are no clear lifesaving benefits to detecting past infections — and obvious risk of harm if the tests give misleading results.
Though much remains unknown, the problem is not that the human body fails to mount some protective antibody response after infection. There’s evidence that it does. It’s that the test results are not what the marketing has led people to expect. They don’t measure a clear indication of past infection.
Recent news reports announced that these tests were “wrong half the time,” attributing this to a CDC report. The big problem reported by the CDC is false positives, and the fact that real positive cases are rare enough that the majority of positive tests could be incorrect.
But the issue is more complicated than that. There’s a misunderstanding of what a false or true positive means. Human blood is full of antibodies left over from different viruses that can easily trigger a test designed to pick up antibodies for Sars-CoV2. The tests work by taking proteins — usually one kind of protein — from the virus and placing it on a surface. Once a blood sample comes in contact with the test strip, any antibodies to the virus should stick to those proteins.
Duane Wesemann, a Harvard professor who is studying immunity to the coronavirus, says he uses more complex tests in his laboratory that can measure the quantity of antibodies and look for one that binds to different parts of the virus.
When applied to people who who’ve had a documented infection — in other words, they tested positive while symptomatic — their blood shows lots of antibodies that latch onto several different proteins associated with Sars-CoV2. People who haven’t had a documented infection still sometimes make antibodies that bind to proteins associated with the virus, and are thus capable of triggering a positive response in some antibody tests. Those people might have had a past, under-the-radar infection of Sars-CoV2 … or not
Test results can also be complicated by the fact that there are several classes of antibodies, including the faster-responding IgM antibodies and the slower and more
persistent IgG antibodies. IgM antibodies not associated with Covid-19 can nevertheless potentially stick to the viral proteins in a test and register as positive.
“There are so many tests and so many levels by which things become positive,” Wesemann says. “That’s why this is such a mess.”

—Bloomberg