The title above is that of an article by me published today in the Mises Wire. It deals with the problem of false positive tests for COVID-19 that will be found if a large population is tested which has a low prevalence of the virus. It also deals with the issue of frequent tests of athletes who likely have a low prevalence for the disease. Frequent testing of a very low prevalence group will eventually yield a false positive test.
My article does not discuss the issue of antibody testing that will soon be initiated. These tests will also have sensitivity and specificity values less than 100%. Thus, there also will be false positives and false negatives for these. So don’t be surprised when patients said to have recovered from COVID-19 are found to be antibody negative. They could have been misdiagnosed (ie, had a false positive test for the virus) and thus be antibody negative because they never had the disease to begin with. Moreover, the test used to diagnosis the presence of antibodies could lack sufficient sensitivity and specificity to detect them. Finally, patients who never had the disease could be false positive for antibodies.
Another issue is that the flu season is just a few months away. It’s very likely that everyone who gets the flu or flu-like symptoms will be tested for COVID-19. Doing so will enlarge the tested pool while diluting the prevalence of the coronavirus, which will increase the number of false positive tests. Angst will surely follow!
The CDC has recommended that all at risk patients get the flu vaccine to reduce the combination of flu and COVID-19 in the same individual. The recent paper in the Annals of Internal Medicine showing that flu vaccination does not reduce hospitalizations or mortality in the elderly does not offer much solace.
These issues of testing are just part of the guidance as to how we should respond to this epidemic. Shutting down for a protracted time is social and economic suicide. As I’ve said here before, we’ll have to figure out how to coexist with the virus until herd immunity kicks in or until a safe and effective vaccine is devised.