Making sense of the data: Number of tests per positive case

An occasional series on the how and why behind the various charts on the Coronavirus in SA Dashboard, and some ideas on what we can, or can’t, learn from them.

The Covid-19 data released daily by the Department of Health has always been very limited. Most days its just the top-level numbers (new infections, deaths and recoveries). To build up a more detailed picture of the progress of the pandemic we collect every piece of data we can find and we combine these into charts that we hope give some insight into the situation beyond just the number of new confirmed infections reported each night.

Sometimes the motivation for adding a particular chart may not be immediately obvious. And often it may not be clear what the chart is meant to illustrate. In this new series we’ll look at a particular chart or charts and try to explain why we think it’s relevant and what we might learn from it.

Today’s chart: The number of tests per positive case

We added this chart back in early April. Like many of the charts on the dashboard this one doesn’t offer any definitive answers but it does offer a specific way to track data over time and hopefully raise some questions that push us to look beyond the day-to-day reporting on coronavirus in South Africa.

The chart shows the average number of tests required to return a positive case. As we can see this number has been coming down steadily since the start of the lockdown in SA on 27 March 2020.

On the week starting 27 March and ending 2 April, there was on average one positive case for every 51 tests done. By the week ending 2 July, the average was one positive for every five tests done.

A low number of tests per positive case could suggest that a country is not testing widely enough to have a reliable picture of possible infections.

For example, testing only people who present with symptoms at a hospital will likely return a high proportion of positive cases (or low number of tests per positive) while testing randomly in a community is likely to return a higher number of tests for each positive.

At present, only 24% of the tests done in South Africa are a result of community screening and testing, according to the National Institute of Communicable Diseases daily Covid-19 reports.

The World Health Organisation has suggested 10-30 tests per positive case as a suitable benchmark of adequate testing. This week (3-9 July 2020) this is down to an average of just 4 tests for every positive case.

For more on the testing positivity rate this video explainer is a good start.

There are many issues around testing in South Africa and there have been significant backlogs in testing at times, so these numbers need to be viewed with those issues in mind. But presenting the results as a seven-day average we can adjust for some of the underlying issues. And now, 18 weeks since we started tracking these numbers, the pattern is very clear and the number of tests required to produce a positive cases is steadily dropping.