from February 13, 2020
Don’t be alarmed at first glance.
There appears to be a large jump in cases overnight. This is strictly due to the way that cases are being identified in Hubei province in China. Most testing has been through RNA tests, but results can take days. Hubei province decided to use CT scan to look for lung infections in order to start treatment earlier. This accounts for the large jump in cases. A similar thing occurred in the US in 1993 when the definition used for surveillance for AIDS changed, resulting in what looked like a massive jump that year.
A change in the testing methodology impacts two important variables: sensitivity and specificity. There are technical definitions for these but I’ll try to describe them in a way that makes them easier to understand.
In this case, switching to CT scan increased sensitivity. This essentially means that the test is more likely to capture cases. That’s why there is a large rise in the number of cases.
The other things that happens in this case is a decrease in specificity. A CT scan will identify any type of lung infection, not just from COVID-19. This is what is also known as a false positive test.
The takeaway is that this not something to get more concerned about. It makes sense to try to treat those at highest risk.
Disclaimer: This commentary is my own interpretation and does not represent the analysis by the government or my employer. The data is from the Johns Hopkins University’s Center for Systems Science and Engineering.