There will be no "antibody passports" for a while. Even if an antibody test has a low (say 5%) false positive rate, if YOU get a positive test, it may only be 50/50 (or less) that you actually have antibodies. WTF? (1/n)

It comes down to the false positive rate versus the positive predictive value. The FPR is how often a test comes back positive in a group of people WITHOUT antibodies. For this example, let's say that's 5%. (2/n)

OK - but as an individual, that number doesn't mean a lot. After all, you don't know if you truly have antibodies or not. That's why you're getting the test. (3/n)

What YOU want to know is, assuming your test comes back positive, how likely are you to REALLY have those protective antibodies. The intuitive answer, 95%, is wrong. Easy math incoming: (4/n)

Let's say the population rate of prior covid infection is 5%. Out of a group of 10,000 people, 500 will have had COVID. Let's assume the ab is 100% sensitive - it detects ALL 500 people who had COVID already. (5/n)

What about the 9,500 people who haven't had COVID yet? Well 5% of those will get a false positive antibody test - 475 people. (6/n)

So we just tested 10,000 folks and found 975 "positive" tests, but 475/975 (49%) never had COVID. (7/n)

That's right - if you were one of those positive peeps, it's basically 50/50 that you are protected, even though the test reports a 5% false positive rate. (8/n)

The critical thing here is that underlying true infection rate (which may be near 0 in some places, but as high as 25% in NYC for example). The lower that is, the higher the chance of a falsely-reassuring antibody test (9/n).

It works on an individual level too. If you never felt sick at all, it's less likely you had COVID, so you can imagine you're part of a population with a low underlying prevalence - high I of being falsely reassured (10/n)

But if you had classic symptoms - a horroble illness with fevers, SOB, close contacts with COVID, etc, it's much more likely that the antibody test is accurate. (11/n).

Sorry to rain on parades here. But don't celebrate your positive antibody test too much. It may not be as protective as you think. (12/12).

P.s if we want antibody passports to be a thing, we need a test with a false positive rate < 1%. It's ok if we miss folks who really did have protective antibodies but not ok to falsely reassure people who are still at risk.