Thanks.
Another lie is that the tests give a high rate of false positives. Because of the inaccurate tests, we can never reach zero cases until a better test is found:
Problems with test accuracy are likely to be more of an issue globally. The current US Centers for Disease Control test kits can generate up to 30 per cent false positives even in their best laboratories. Highly accurate tests can prove costly – more than £100 per test. So, we shouldn’t be surprised that in poorer countries, highly questionable cheaper alternative tests, which cost less than £3, have been distributed and used. A recent BMJ review reported that the specificity of PCR tests could be as low as 95 per cent, as PCR test performance can be much worse in low prevalence community settings. This would mean that, in our hypothetical of 10,000 tests, we’d have 500 false positives amongst the eight genuine positives. So the hundreds of false-positive Covid-19 results would dwarf the genuine results – meaning an apparent surge in infections that is not followed by a corresponding surge in hospital admissions or deaths.
At very low prevalence, the proportion of people with infection falls and the numbers of falsely misdiagnosed increases. If Covid-19 completely disappears, then of our 10,000, no one will be infected. If you have followed the reasoning so far, you will have worked out this means that ten people would still be wrongly diagnosed as positive and the official data would show a national Covid-19 prevalence of 0.1 per cent. This is why understanding the accuracy of tests in the population that they are applied to matters: going off current testing practices and results, Covid-19 might never be shown to disappear.
Professor Carl Heneghan is director of Oxford’s Centre for Evidence-Based Medicine.
How many Covid diagnoses are false positives? | The Spectator
