I set up tests ...it still worries me me. (even though it is from Abbott who are a re pretty good diagnostic company)
It is to do with the specificity or sensitivity of the tests. There is a very important difference. There test is 99% sensitive....but no mention of specificity
SENSITIVITY: The true positive rate, is the number of real positives that are picked up in a test
SPECIFICITY: This is the proportion of the real negatives that are picked up. ...Inversely it means how many false positives are there
You can set up any test to get the "results you want". So set the criteria for testing positive very high. You pick up 70% of positives and no false "positives" you can be sure if you get a positive , it is right...but you miss 30% of positives...BAD... But the specificity is 100%...because a positive is positive (100% correct)
Set it up another way...very low cut-off...you get all the positives, butsay you get 40% of the healthy population (who really don't have the disease) as positivity. The sensitivity is then 100%, everyone who has a positive test has the virus/antibody. But 40% of the population think they have the virus/antibody but don't.
Intuitively you think the second criteria is good... But actually for the antibody test it is not...really it is not.
So you give those people a knowledge (or a paper passport) that they have had the virus, and they have antibodies that protect them and they don't...You send them off into the wide world to catch the virus. Think of putting a nurse back onto a Covid ward with the knowledge she was safe ...and she wasn't. She had a cough...but it was not CoVid, because she had had a positive antibody test
This is apparently what happened with the China kit...low cut off...loads of positives in the kit...but a lot of them were "false" positives