Testing for COVID-19

The majority of COVID-19 testing in the UK to date has been Polymerase chain reaction (PCR) testing which is lab-based and tests for the presence of SARS-CoV-2 virus (rather than testing for the body's immune response or antibodies). This data shows the number of people who received at least one PCR test in the previous 7 days, as well as the percentage of people who received a PCR test in the previous 7 days who had at least one positive PCR test result (the positivity rate). People are only counted once in a 7 day period.

Lateral flow device (LFD) virus tests are another type of COVID-19 test that look for the presence of SARS-CoV-2 virus. These are also swab tests, like the PCR tests, although they give results in less than an hour, without the need for laboratory processing. LFD tests are generally used for mass and routine testing of people who do not have COVID-19 symptoms. They are aimed at identifying individuals who may be infectious asymptomatic carriers and potential transmitters of the virus so these people can isolate and take the virus out of circulation in the community.

Area PCR Positivity rate (weekly percentage of individuals tested who test positive for COVID-19)

Data source: https://coronavirus-staging.data.gov.uk

It is important to consider test positivity with caution and alongside the other measures such as confirmed case numbers and the information available on those being tested.

A higher percentage of positive tests could suggest higher transmission, and perhaps that there are more people with coronavirus in the community who have not been tested yet, which could further indicate a need for conducting a higher number of tests. However, it could also be that a change in positivity is related to different groups of people coming forward for testing. It is likely a mixture of several reasons.

How testing has changed over time

The overall number of virus tests conducted (using lab-based and rapid tests) each day is only available at a national level.

For local areas (West Sussex or its districts and boroughs) we can only see the number of individuals tested using PCR tests in a seven day period or the total number of LFD tests conducted each day. These two measures cannot be combined into a single metric for how many people are being tested and must be considered separately.

The next figure shows how the rolling 7-day number of individuals tested has changed over the course of the pandemic, and particularly how testing ramped up towards the end of 2020.


Data source: https://coronavirus-staging.data.gov.uk

The next figure shows the proportion of individuals with a positive result for the same area as in the above barchart. Comparing these two charts helps to contextualise testing. It shows that whilst the number of tests in the early stages of the pandemic were low, a much higher proportion of tests were positive (in some cases around half of tests). In more recent months, many more tests are conducted but fewer of those are positive.

Data source: https://coronavirus-staging.data.gov.uk

Whilst the number of individuals tested using the lab-based PCR tests is available for the whole of the pandemic (since February 2020), LFD testing in England only became widely available from the 21st October 2020.

As such, the time period for the next figure (number of LFD tests conducted) is much smaller (for the last few months only). Note also that data are available for very recent days (up to the day before). Whilst test results are much quicker to obtain for LFD tests compared to PCR tests, it is expected that the totals for the last few days will still not be complete due to delays in getting results into the national data systems.


Data source: https://coronavirus-staging.data.gov.uk

Note that the PCR and LFD figures are not directly comparable because of the different date ranges.