In its first account of statewide antibody testing, the Florida Department of Health reported Friday that about 4.4 percent of more than 123,000 tested came back positive for signs of an infection, providing a snapshot of how the disease might be spreading.
But the report warns that the accuracy of commercial lab results included in the mix is not known and experts say even results from approved tests could have a high degree of inaccuracy.
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“You're in Miami. I'm in Switzerland. If both of us have a positive test today, the probability that I am positive and you are positive is actually very different,” said Andrew Azman, an epidemiologist with the Johns Hopkins Bloomberg School of Public Health and co-author of a recent paper in the journal Science Immunology on how antibody testing should be used to deal with the coronavirus outbreak.
That’s because the rate of false positive results could be higher, or lower, depending on the prevalence of the disease from one place to another. Even among different groups of people being tested, like healthcare workers who have a higher rate of exposure, probability rates differ.
“Basically if you're in a place that has low prevalence or low risk before you take a test and your risk is very, very low, if your test is positive, it's most likely a false positive,” Azman said.
But in an areas where the disease is widespread, he said, where, for example, half the population is infected, then it’s likely truly positive.
Last month, in an assessment of its 12 approved tests, the U.S. Food and Drug Administration created a calculator to better understand the accuracy. If the rate of prevalence is just 5 percent, the tests are only expected to be correct half the time. But if the rate of prevalence doubles to 10 percent, accuracy increases to more than 70 percent.
“It's not magic, but it is not intuitive,” Azman said.
The state increased its antibody testing last month as it began lifting shutdown orders and began reopening beaches and other public places. Gov. Ron DeSantis suggested the testing could be an indication of immunity, but health experts have warned that it's not yet clear what kind of immunity antibodies might provide. The body begins producing antibodies when it detects an infection. But it's not yet clear whether the antibodies from a COVID-19 infection last long enough to prevent a second infection.
And neither the FDA nor Azman say the tests are sufficiently accurate for individual use. They can even pose risks.
“There's probably more harm that can be done than good with individuals, because there is likely to be some degree of what we call risk compensation, where somebody believes that they are protected and then they may take extra risks that they wouldn't otherwise take,” Azman said.
Banyan Medical Systems has begun providing drive-up antibody testing in several Miami-Dade County municipalities, including Aventura, Surfside and Bal Harbour. Vice President Michel Koopman declined an interview request through a representative, who asked for questions in writing first.
Coral Springs and several Broward County cities have also started testing workers. Coral Springs Fire Battalion Chief Chris Bator said Coral Springs will repeat the tests over the next three months to track employees. Results are being provided to the state as part of an ongoing study, he said. When asked about the study, Florida Department of Health spokesman Alberto Moscoso said the state was looking into a broader study, but provided no additional details either about the study or the results released Friday.
To better understand antibody testing, WLRN asked Azman to explain its best use. The following is an edited excerpt:
Since you've studied other diseases and the use of anybody testing, can you kind of put where we are now in context? Antibody testing now is a surveillance tool, not a diagnostic tool. So sitting in a line at a drive-thru is not what's being recommended by the experts.
And especially with those drive-thrus, which are by no means perfect tests and less perfect than some of them, more like what we call a ELISAs, which take a bit more time. And so if you're getting like these rapid results, you're getting even less perfect results, which are even harder to interpret than kind of the more gold standard approach.
Can you talk about immunity? Because I think that people think that if they get an antibody test, it will tell them, oh, I've been infected, maybe I didn't have symptoms and maybe this confers some kind of immunity.
This virus didn't exist six months ago, or a little bit more than six months ago, in the world. And so we're learning a lot about this as we go. And I think the fundamental issue we have right now is that people want a marker of protection and want to know whether there's evidence in their immune system that they are protected. And we don't know if anything in your immune system actually correlates with protection. We don't even know if you're infected today if you can get reinfected or not.
We're very worried with a lot of these tests about cross reactivity with some other coronaviruses. So other coronaviruses that circulate more commonly, like the common cold and things like that, if you have antibodies to them...they may actually give you false positives to some of these tests.
But what you think you can do is to use it as that sort of surveillance tool? In your paper, you said diagnostic testing and contact-tracing would always be the cornerstone of containment, but that the anybody testing could provide the population's susceptibility and intensity of...
Transmission. It's certainly a really important surveillance tool. And I think that this epidemic will bring serosurveillance to more of the mainstream.
If we're going to be collecting probably unprecedented amounts of blood from people around the world for this, from many, many, many countries, and drops of blood have a lot of information in them, not just about COVID. And I'm quite hopeful that this mass response to track the COVID epidemic will actually give us the ability to really look deeply at populations and antibody profiles and really understand many, many different pathogens, a lot better.
From a good public health standpoint, how do you want these tests administered?
From a good public health standpoint at this point, the individual use of these tests is not...there's not a great individual use case for these tests right now. And I think there's probably more harm that can be done than good with individuals, because I think that there is likely to be some degree of what we call risk compensation, where somebody believes that they are protected and then they may take extra risks that they wouldn't otherwise take and engage in behavior that they wouldn't otherwise do.
Once we have some sense that this is correlative — if one of these antibodies is a correlative to protection —we can then move to a phase where we set a very, very high bar for these tests, where you have to have a really, really high antibody level for us to say you're positive, which would reduce the false positive rate. But there would be a lot of false negatives. So a lot of people who would test negative would actually have antibodies. But I think that for individual use, that's a trade-off we probably have to make, where a select few will know that they're positive. And they're really positive. But a lot of people will be in-between.