Just as it is not informative to look at case counts in isolation it is also not informative to look at the number of tests in isolation. We need to see the number of tests in relation to the size of the outbreak: countries with large outbreaks need to do much more testing to monitor the spread of the pandemic than countries where the disease is under control.

Based on this principle we calculate a quality metric for the case counts that answers the question: what share of tests in a particular country confirm a case of COVID-19? 

This metric is referred to as the positive rate or positivity rate.

The world map here shows this data – it is exactly the same data shown as the line color in the chart above.

By moving the time-line below the map you can see how this metric has changed around the world; by clicking on a country you can see how it changed in every country.

The world map shows enormous differences across countries.

Countries that do very few tests per confirmed case are unlikely to be testing widely enough to find all cases. The WHO has suggested a positive rate lower than 10% – but better lower than 3% – as a general benchmark of adequate testing.

The countries that have a positive rate below 3% are shown in shades of blue. Those that have a positive rate higher than 5% are shown in shades of red.

In countries that test very little in relation to their outbreak – shown in shades of red in the chart – many cases are likely to go unreported. In these countries, the number of confirmed cases indicated may represent only a fraction of the total number of cases.

Source : Our world in data