Like other COVID-19 figures, there are nuances in definitions for testing.
This article discusses measuring total tests and people tested in the UK.
The UK government has four different ‘pillars’ of tests for SARS-CoV-2 and antibodies. This is the virus that causes the COVID-19 disease:
- Pillar 1: swab tests by Public Health England labs and NHS hospitals. These tests are for people with clinical need, to test if they have the virus.
- Pillar 2: swab testing of the wider community, if people have the virus.
- Pillar 3: antibody tests to show if people have already had SARS-CoV-2.
- Pillar 4: blood and swab testing for national surveillance. This is the pillar for tests by statisticians and academic partners, estimating prevalence.
There are counting differences between Wales and the other nations.
For the first pillar, there are different data sources for each nation:
- England: NHS England and Public Health England.
- Wales: Public Health Wales. This figure includes some community tests — where NHS labs process those tests.
- Scotland: the Scottish Government.
- Northern Ireland: Department of Health of Northern Ireland.
The second pillar statistics comes from commercial testing partners. The Welsh figures excludes some community swab tests, processed in NHS labs.
The Department of Health and Social Care collates these figures. Descriptions of these figures are correct as of 28th September 2020.
Tests processed and tests made available
Earlier in the pandemic, the UK government had a count of ‘daily tests’. When home kits started, the measure added tests carried out with tests posted out.
Sir David Norgrove (UK Statistics Authority) wrote to the Health Secretary:
This distinction is too often elided during the presentation at the daily press conference, where the relevant figure may misleadingly be described simply as the number of tests carried out. There are no data on how many of the tests posted out are in fact then successfully completed.
The Department of Health and Social Care developed two headline measures for testing:
- Number of tests made available: tests in the central programme count when processed. For any tests that go outside that programme: those tests count when they leave.
- Number of tests processed: counted at the time of processing within the lab. Tests are not double-counted.
In both measures, the DHSC report nose and throat swab tests as one sample.
In total, the number of tests processed will be lower than tests made available. That is due to: delays in returning tests, invalid returns, and people not sending tests back. Due to when each measure counts, processed tests can exceed tests made available on a single day.
Seeing statistics for the four nations is important. The UK-wide figures include all types of test. National surveillance tests (‘pillar 4’) are not shown by nation. There are differences in how testing pillars show for each nation:
- England and Wales: pillar 2 tests are only shown after 14th July. From 8th August, the DHSC reports all pillar 3 tests as England. In Wales, some ‘pillar 2’ tests count under the first testing pillar.
- Northern Ireland: pillar 2 tests are only shown after 3rd July. There was no weekend reporting between 4th July and 10th August.
- Scotland: before 7th July, the DHSC used statistics collected from contractors. Those figures did not include processed home-testing kits. After 8th July, the source is the Health Protection Scotland ECOSS system. The published statistics now include processed home kits.
Discontinued time series
The Department for Health and Social Care no longer updates the daily statistics page.
The page recommends the Public Health England dashboard. Testing figures have weekly — rather than daily — updates.
There are a number of discontinued time series:
- In-person tests processed: ‘pillar 2’ tests which were processed in-person. For example, these tests would be carried out at mobile test sites.
- Delivery tests sent out: community tests were sent out. The number of ‘pillar 2’ tests made available is the sum of in-person processed tests and these deliveries.
- Number of people tested: a unique count of people tested each day. The DHSC no longer produced this figure after 2nd July.
At present, we cannot calculate the daily positivity ratio. That is the number of confirmed cases divided by the number of people tested.
This is important to understand. A greater number of confirmed cases could come from increased testing alone. The goal of a testing programme is to identify carriers of the virus. That way, those people can help control the viral spread.
Without this measure, we rely on: confirmed cases divided by the number of processed tests. Changes in that measure could be due to differing numbers of tests for each person.
Within the four nations
There are separate reports about tests for England, Wales, Scotland, and Northern Ireland.
NHS Test and Trace (England)
These weekly reports provide management information for the test and trace programme. The experimental statistics are for England, and may change in future.
For the first two testing pillars, there are two measures:
- Number of people tested: if a person has no positive result, they are in the week and pillar of their first test. If they have a positive result, that person moves to the week and pillar of their first positive test.
- Number of people testing positive: in the week and pillar of their first positive result.
These definitions ensure the total number of people tested is since the launch of Test and Trace. The problem is past weeks need revision. Retested people move if they later test positive.
The weekly reports include UK testing capacity, tests sent out, and tests processed.
Public Health England
Public Health England publish weekly surveillance reports on COVID-19. The focus is on confirmed cases. The report also includes a weekly positivity ratio. For the two testing pillars, that ratio is for: total, sex, and sex by age group.
Public Health Wales
On their daily dashboard, Public Health Wales measures:
- Testing episodes: an individual in a period of 42 days. If someone has two tests — 43 days apart — that is two testing episodes. If any tests in an episode are positive, only that result is shown.
- Positivity: Cumulative cases divided by cumulative testing episodes.
Public Health Scotland
The number of people newly tested refers to the number of individuals in Scotland who have been tested for the first time for COVID-19 on the previous day. Note that each person is only counted once regardless of repeat tests. This means that where someone tests positive after receiving a negative test result on a previous day, they would be counted in the new positive cases for that day but in the number of people newly tested on the day they first received a test result.
The daily measure is for Scotland, by age group and sex, and by deprivation segments. The dashboard shows the positive ratio is for NHS boards and local authorities.
Public Health Agency (Northern Ireland)
Public Health Agency have a Power BI dashboard. The measures are daily, but the updates are now weekly.
Alongside the number of ‘pillar 1’ and ‘pillar 2’ tests, there are more measures:
- Individuals tested: the number of people with a lab test for SARS-CoV-2. If one person has many tests, the dashboard reports the most recent positive result.
- Cumulative individuals tested: the total number with a SARS-CoV-2 lab test since February 2020.
- Positive proportion: the proportion of lab tests which came back positive.
PHA reports the change in the total number of individual tests.
As Georgina Lee (C4 FactCheck) finds, no health agency reports a daily number of people tested.
Different parts of the United Kingdom have different testing measures. Analysts should be aware of these differences, and understand the strengths and limitations.