Also be aware that there are false positives, especially significant with these antibody tests, so with the low numbers of positives it's extremely important to evaluate if the claimed values are more than noise artifacts of the tests themselves. It is also important to be aware of the scenarios for which the use of the apparent test results is not reasonable.
Moreover, here's what happened in the UK a few weeks ago:
https://www.theguardian.com/world/2020/apr/09/uk-government-...
"None of 3.5m home tests ordered have so far been accurate enough to detect coronavirus immunity"
UK got 3.5 million(!) unusable antibody tests.
https://www.bmj.com/content/369/bmj.m1449
"John Newton, Public Health England’s director of health improvement, said:"
"A number of companies were offering us these quick antibody tests, and we were hoping that they’d be fit for purpose, but when they got to test, they all worked but were just not good enough to rely on.
“The judgment was made [that] it’s worth taking the time to develop a better antibody test before rolling it out, and that is what the current plan is.”"
"Newton told the committee that the tests trialled so far had lacked sufficient sensitivity to identify people who had been infected. “We set a clear target for tests to achieve, and none of them frankly were close.”"