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sorry to reply to myself, but wanted to make a follow-up post...You're right, that's what the result would indicate. In practice, the numbers are all estimates, so if the false positive rate is supposed to be 40 people, it's really ballpark 40, so it could be 10, it could be 100.
I don't know if they tried to do any statistical correction in the study. It's possible that 10% tested positive and then, using a known false positive rate of 5% for their test, corrected it down to 5%. Either way, it's a lot of uncertainty.
from digging around this is the only thing I could find, which indicates a much lower false positive rate for the test they did than the others that have been out. They also did a better job at randomizing than prior studies:
How reliable are the antibody tests?
Premier Biotech, the manufacturer of the test that USC and L.A. County are using, tested blood from COVID-19-positive patients with a 90 to 95% accuracy rate. The company also tested 371 COVID-19-negative patients, with only two false positives. We also validated these tests in a small sample at a lab at Stanford University. When we do our analysis, we will also adjust for false positives and false negatives.
Can you talk about how people were selected for this study?
Participants for the USC-L.A. County study were recruited by the market services firm LRW Group using a large proprietary database that ensures factors such as age, race and sex are part of the random selection. For the first testing that took place on April 10 and 11, USC and the L.A. County Department of Public Health identified six sites for drive-thru testing. Our plan moving forward is to test a different group of 1,000 randomly selected people every several weeks.
If you have questions about USC COVID-19 antibody testing in L.A. County, he has answers
Neeraj Sood of USC Price describes the antibody testing taking place in L.A. County and explains what it will teach us about the coronavirus.
news.usc.edu