The Moving Curve: Episode 25

Hello and welcome to The Moving Curve. I’m Rukmini, a data journalist based in Chennai. Every night on this mini-cast, I consider one question around the novel coronavirus epidemic in India. Tonight I’m considering this question — can test positive rates tell us anything useful at this point?

It’s Day 79 of the novel coronavirus epidemic in India, and we are reporting 14,792 cases with 488 deaths. Last night, I talked a little about what we can safely say about that second number — the number of deaths — based on what we know about the likely scale of under-reporting and misreporting of deaths. Tonight I want to talk a little about the test rates which are producing that first number, the number of confirmed cases.

This is what the current situation on testing is: in all, India has tested 3.5 lakh people so far. We tested over 35,000 samples today, and over 2,100 of these tested positive: https://icmr.nic.in/sites/default/files/whats_new/ICMR_testing_update_18April_9PM_IST.pdf

One indicator that the ICMR’s head scientist Dr Raman Gangakhedkar has mentioned more than once is the “test positivity rate”. This is the ratio of confirmed cases to total tests conducted, expressed as a percentage: https://economictimes.indiatimes.com/news/politics-and-nation/india-conducts-24-tests-for-one-coronavirus-positive-case-icmr/videoshow/75185393.cms?from=mdr

I thought of doing this episode on this issue partly because of a piece I read in The Atlantic that I’ve linked to which was worrying about the US’s high test positivity rate, which really proves to me that you can argue that this indicator means something and it’s opposite: https://www.theatlantic.com/technology/archive/2020/04/us-coronavirus-outbreak-out-control-test-positivity-rate/610132/

Ultimately, the most useful thing might be to agree that the test positivity rate tells you more about testing strategy than about prevalence. As it happens, India’s testing positivity rose today. Is that good or bad news? You’ll have to take your pick.

Thank you for listening. This episode was edited by Anand Krishnamoorthi. Tomorrow — a new question.

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I am a data journalist based in Chennai, India.

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