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The FDA has approved the first antibody test for COVID-19, from Cellex. It theoretically tells you if you& #39;ve had it & are, as far as we know, immune for some time.
Sensitivity is 93.8%, specificity 95.6%. Sounds great, right?
Well, sort of. (1/6)
Sensitivity is 93.8%, specificity 95.6%. Sounds great, right?
Well, sort of. (1/6)
If only a small % have actually had COVID-19 (our best guess now) a "positive" antibody test isn& #39;t that likely to mean you& #39;re immune.
If only 4.5% of U.S. has had COVID-19, + test only means ~50% chance you really had it. With lots of uninfected, lots of false +s. (2/6)
If only 4.5% of U.S. has had COVID-19, + test only means ~50% chance you really had it. With lots of uninfected, lots of false +s. (2/6)
If 10% were truly infected, a positive Cellex antibody test has a 70% chance of being right. If 30% were infected, a positive test is right 90% of the time. This happens bc when more people were sick, true positives overwhelm false positives. But that& #39;s not our situation. (3/6)
So this is test may not be that useful for saying "Zach, you are immune; Jen, you aren& #39;t." It might be wrong as often as it& #39;s right. Mistakenly telling someone they& #39;re immune & clear to return to society...you can see the problem if we do that on a large scale. (4/6)
There are simple equations to correct for this on a population level. So this test is still *very* useful for helping us figure out what % of people have been sick in different areas. And it& #39;s the best we& #39;ve got; deploy it! But realize what it will & won& #39;t reliably tell us. (5/6)
Running the test 2x & only telling someone they& #39;re immune if they get 2 +s *might* help reduce false +s, depending on the error source. If it& #39;s anything systemic - say it& #39;s detecting antibodies from a similar virus that don& #39;t grant immunity to COVID-19 - no good. @KevinMalogna
Bonus tweet 1: A positive test would be more likely to mean you& #39;re truly immune if you& #39;re in a high-risk group - healthcare worker, had COVID symptoms, family member had COVID - bc prevalence of infection in these subgroups is higher. So test may be more useful for these folks!
Bonus tweet 2: We might be able to reduce false +s/increase the chance a + test is right by using this test as a screener and another slightly different antibody test to confirm, if they wouldn& #39;t both show false + for the same systemic reason like antibodies to a similar virus.
If you& #39;d like to nerd out on this stuff further, here are more details on the various statistics we& #39;re talking about and why this test may not do some of the things we need when the prevalence is low.
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