Antibody tests have the potential to be a game-changer. But many of them aren't all they're cracked up to be. In this article (and this thread) I explore at the impact of false positve and false positives in COVID-19 tests. theconversation.com/coronavirus-su… @ConversationUK @UniofBath 1/10 
Cellex have had a test approved by the FDA. If you have antibodies against COVID-19 their test will tell you this correctly 93.8% of the time (this is the test’s “sensitivity”). If you don’t, it will get this correct 95.6% of the time (this is the test’s “specificity”). 2/10
Getting the correct result more than 90% of the time sounds pretty good. But there's a catch. To find out why lets consider testing 10,000 hypothetical individuals. The WHO suggested recently that as few as 3% of the global population may have had COVID-19 and recovered. 3/10
This means that 9,700 of the 10,000 tested will not have had the disease and only 300 will have. Of the 300 recovered patients, 93.8% - or 281 - will be correctly told they have antibodies against the disease. 4/10
Of the vast majority (9,700) of people who haven’t had the disease, 4.4% - or 427 - will be incorrectly told that they have had the disease and recovered. Far more people will receive false-positive results than true-positive results. 5/10
Up to 60% of those released back into the workforce could be at risk of infection themselves and unknowingly spreading the disease to others, sparking a second wave of the epidemic. If the true prevalence of the disease is as low as 1% then this figure could rise to 80%. 6/10
Re-testing those people who tested positive on the first test and issuing immunity passports only to those who have received two positive results could reduce the proportion of false positives to below 7%. But... 7/10
double-testing only works if the results of the two tests are independent. If the reason for the false positives is systematic - detecting antibodies from other coronaviruses, for example, then there is no reason to believe a second test will do any better than the first. 8/10
Understanding the startling rates of false positives and false negatives for tests that seem, on the surface, to be quite accurate could have profound consequences for health policy as we travel deeper into this pandemic. 9/10
Failing to do our mathematical due diligence has the potential to take us past the tipping point beyond which the epidemic starts to grow again, leading to even more avoidable deaths. 10/10
