Here's a new problem for India's testing data: states like Delhi are conducting both RT-PCR tests and antigen tests, but reporting positive test numbers combined. Antigen tests have much more false negatives, so start taking falling test positivity rates with a big lump of salt.

I wrote about it here for @IndiaToday: https://www.indiatoday.in/diu/story/why-delhi-turnaround-fight-coronavirus-covid-19-cannot-face-value-1698623-2020-07-09

From July 3-6, Delhi conducted more antigen tests than RT-PCR tests, so that immediately makes its falling test positivity rate very difficult to celebrate.

The problem is bigger: "test positivity rate" isn't just a cute number nerds like looking at. It directly means that a greater share of the testing is now being done via a less sensitive test, so new patient discovery should be seen with some scepticism.

The health ministry requires every negative antigen test to be re-conducted via RT-PCR. Zero detail on that from any state. But that doesn't take away from the issue - discovering new positives the more your testing skews towards antigen testing.

Adding mass antigen testing (cheap, quick) to your RT-PCR testing is a public health improvement. Replacing RT-PCR tests with antigen tests is not. Reporting it all so opaquely is an all-round disaster.

Kerala is the only other state that I found reports its antigen tests. These tests made up less than 1% of their total tests so far. They too do not report positives separately, but the scale of the problem is much smaller than Delhi, which is leaning so heavily on antigen tests.

Last tweet of this thread: please demand that your state releases antigen testing (tests conducted daily AND positives) data separately. It matters.