While increased testing will find more cases, this simple logic fails to show the full picture.

Over the last few weeks, the UK has seen an increase in test-positive coronvirus cases. While the Government mulls imposing new national restrictions, many are still at a loss as to why. 

One of the biggest fleeting statements that people have been making is that ‘the number of test-positive cases will obviously rise when you do more tests’. While there is logic to this statement, it fails to account for one key measure: the positivity rate.

The positivity rate is the proportion of tests that are returning a positive result. It’s a much more insightful measures of whether the epidemic is growing in the country or not, accounting for the increase in testing.

At the start of the epidemic in the UK, the number of tests being carried out was relative lower and the true nature of the spread of the virus was therefore unclear.

However, at the start of August, the number of pillar 1 and 2 tests processed (swab tests processed by the NHS and commercial partners) averaged around 125,000. This has grown to around 225,000 today.

To put it simply: if you carry out 1,000 tests and one returns a positive result, you have a positivity rate of 0.1% If you then increase testing, you will of course pick up more positive cases but you’ll also pick up more negative cases too. Let’s say we increase out testing to 100,000. If the positivity rate remains the same, we would expect around 100 positive tests.

If the rate of infection and the spread of disease remained the same, or was coming down, you would expect the positivity rate to come down as you increase the number of tests.

As you can see from the chart however, the positivity rate is rising – meaning the increase in test-positive cases is rising despite the recent increase in testing.

The positivity rate in England has risen from about 0.5% at the start of August to about 1.5% in mid-September.

While there has been a recent drop in the positivity rate over the last few days, it remains to be seen whether this will rise as tests from those days are processed.

The Government will be hoping that the ‘rule of six’, announced last week, and more localised restrictions, will start to flatten the rate over the next couple of weeks.

If both cases and the rate continue to increase, the Government’s concern will turn to hospitalisations (and unfortunately deaths), with little option but to impose stronger national restrictions.