By Professor Carl Heneghan*
The Spectator, July 20, 2020: Test, test, test said the WHO. And globally, that’s what everyone did: nearly 300 million tests have been done to detect more than 14 million cases of Sars-CoV-2 so far. The thinking goes: turn up, have your test, and if positive, you must have the disease. But that’s far from the truth. When virus levels in the population are very low, the chances of a test accurately detecting Covid-19 are less than 50 per cent – for reasons that are not very widely understood.
There are two issues about tests to get your head around. The first is the sensitivity of the test: the proportion of people who test positive, out of the population who have the virus. The second measure, specificity, is about the proportion of people who test negative, out of the population who should have tested negative. Finding out the actual values for these two measures is tricky. The Office for National Statistics admits they do not ‘know the true sensitivity and specificity of the test because Covid-19 is a new virus’.
Estimates suggest that roughly 80 per cent of infected people will have a positive test (the sensitivity). Based on the latest data, specificity may be as high as 99.9 per cent for those who test negative. I think this is a bit high, but let’s run with it for now.
