Understanding data on coronavirus (COVID-19) testing

The type of swab test processed through Welsh laboratories involves a ‘single dry swab’ taken from the back of the throat. The type of test processed through English laboratories involves ‘two wet swab’ sample collections taken from the nose and throat.

COVID-19 antibody tests are used to find out whether a person has previously had the virus. The antibody test works by taking a blood sample and testing for the presence of antibodies. Antibodies are produced by the body in response to an infection and can usually be found in the blood after around 2 weeks following infection.

There are several ways a person can be tested for coronavirus, depending on whether they are in hospital, a critical worker or a member of the public. And there are many ways that the data on those tests can be presented depending on what it is we’re trying to measure. This guide aims to explain the ways coronavirus tests are counted and some of the common terminology.

A flowchart is provided showing the process for each testing route and how the turnaround times are measured.

Public Health Wales (PHW) publishes a daily surveillance dashboard including data on authorised tests, testing episodes, positive cases, incidence and deaths due to COVID-19. The data in the dashboard are compiled and published on a daily basis to reflect the most recent COVID-19 activity up to the previous day and is used for surveillance purposes. All data presented in the dashboard are provisional and are subject to revisions as more information becomes available.

The Welsh Government brings together a weekly summary of this information, plus additional detail on the number of tests on critical workers and the location that the test sample was collected and turnaround times of tests.

From 13 July, Public Health Wales have combined tests and results processed in NHS Wales laboratories and non-NHS Wales laboratories, which were previously reported separately. Further details are included in the release outlining which tables and charts show tests authorised for both laboratories or separately. The data has been revised historically except for the annex table showing the number of tests authorised in the last 24 hours to 1pm, which reflects what was published on the Public Health Wales surveillance dashboard on a given day.

Different test dates have been used when reporting test data depending on the nature of the data and the use. For rapid surveillance data the authorised test date has been used and is included in the headline figures presented by Public Health Wales, whereas for detailed surveillance over time the specimen date has been used.

The authorised test date is when the result of the test has been authorised and released into the Welsh Laboratory Information System (WLIMS) for clinicians to access and then inform patients of the result. The time taken for a sample to be processed and provide a result varies and does not indicate the number of samples collected or samples processed on a given day.

The specimen date is the date the sample was collected from the patient. Since the time taken to test samples and report the results varies, new cases reported on a daily basis in the headline summary by Public Health Wales may be distributed across a range of specimen dates. The specimen date is used in the charts within the PHW dashboard to show the number of test samples taken and positive samples each day. This is a suitable date for surveillance of COVID-19 over a period of time.

Number of authorised tests per day, as originally reported by PHW

The number of authorised tests per day represents the number of newly authorised test results available in the 24 hours to 13:00 each day. The date that these tests were physically carried out may not be the date that the results were authorised or made available for surveillance purposes.

This data is presented as a snapshot of results authorised over the previous 24 hours and aims to give a rapid surveillance picture in the most timely manner. It gives an indication on the volume of tests authorised within a 24 hour period, however, it may include individuals who may have been tested more than once during a six-week testing episode.

This data is presented on the headline summary of the Public Health Wales Surveillance Dashboard each day but the authorised tests retrospective series is the more up-to-date and complete measure to use. Therefore, data in this series is not revised, whereas the number of authorised tests per calendar day is.

Individuals tested with authorised test result by test date

This data presents the number of individuals tested (testing episodes) with authorised results. The number of testing episodes and positive cases are reported by the specimen date, which is the date the sample was taken from the patient.

This data is used for surveillance purposes and is suitable to compare over time.

Individuals may be tested more than once for COVID-19 for numerous reasons. A testing episode is a six-week period starting from the date of the first sample taken from the patient. Individuals who are tested multiple times during a six-week period are only counted once during that period.

If any of the test results for the individual are positive then that is the result which is presented. If an individual tests positive more than once during the six-week period then this is still recorded as only one new case.

Any tests which occur more than six weeks after the initial test will trigger a new testing episode.

Local Health Board level information is provided to show the different disease trajectories in Wales. This information is based on area of residence and will not necessarily reflect the Local Health Board the patient has been treated in. The age and sex distribution of total testing episodes and total positive cases is also provided for the latest date available.

This data is regularly revised as more tests are authorised and included in the time series. The total number of tests authorised will differ and likely be higher than the number of individuals tested.

The Welsh Government weekly testing summary includes information from the Public Health Wales Surveillance Dashboard which has been outlined above. This includes:

• number of authorised tests per day, as originally reported by PHW
• number of authorised tests per day, retrospective series
• individuals tested with authorised test result by test date

The definitions used in the weekly testing summary are the same as those in the Public Health Wales Surveillance Dashboard which have been outlined above. The weekly summary also includes the following data sets which are not available through the Surveillance Dashboard.

Number of authorised tests per calendar day, retrospective series

This data is more up-to-date and a complete measure to use when comparing daily figures rather than the headline figure presented on the Public Health Wales Surveillance Dashboard. The data includes tests that may not have been initially reported in the rapid headline figure following further reconciliation and quality assurance. The headline figure presented daily is used to give a rapid surveillance picture at 1pm each day.

The number of authorised tests per day represents the number of newly authorised test results available on the calendar date of authorisation. The date that these tests were physically carried out may not be the date that the results were authorised or made available for surveillance purposes.

This data is presented as a snapshot of results authorised over the previous 24 hours and aims to give a rapid surveillance picture in the most timely manner. It gives an indication on the volume of tests authorised within a 24 hour period, however, it may include individuals who may have been tested more than once during a six-week testing episode.

The data presented in the statistical release charts the total number of tests authorised for each calendar day. The accompanying tables present the total but also the spilt between the number authorised in NHS Wales laboratories and in non-NHS Wales laboratories (known as lighthouse laboratories).

Number of tests and cases by critical worker and resident where critical workers are employed

This data presents a snapshot at a moment in time of the total number of tests and authorised results of critical workers and residents where critical workers are employed.

The data shows the number of tests, negative cases and positive cases. This can be used to monitor the positive cases and tests completed of critical workers and residents where critical workers are employed.

The main source of information for identifying that a test related to someone's status as a key worker or resident of a particular setting are the Site and Qualifier fields in the Electronic Test Request (ETR) system. This is only available for tests processed in NHS Wales labs, and the choices available for those fields have changed over the course of the pandemic, in response to changing operational requirements.

Many tests since mid-June 2020 are identified as "Key worker or resident: care home" on the basis of the testing location being "Care Home Test". The UK.GOV care home portal went live on the 15 June 2020.

Some tests are identified as "Key worker or resident: prison or detention centre" on the basis of the referrer or testing location being a prison.

Some early tests are identified as "Key worker: healthcare - other or unknown healthcare" on the basis of the referrer (e.g. an Occupational Health department.)

Some tests prior to Monday 18 May 2020 are identified as "Key worker: healthcare - other or unknown healthcare" on the basis that they were tested at a Community Testing Unit or similar and no other information is available about the status of the individual tested.

The number of tests recorded for critical workers and residents where critical workers are employed is dependent on the critical worker/resident category being recorded in the system when the sample was taken. It is currently not possible to distinguish between key worker and residents for samples processed in non-NHS Wales laboratories so they have been included together.

It is possible there are multiple tests for individuals within this data.

Some of the detailed data has been suppressed to avoid disclosure.

Location of tests

In the Welsh Government’s weekly summary of testing data the location of tests tables presents the weekly number of tests authorised by different testing locations. There is a separate table for tests authorised in NHS Wales laboratories and another for tests authorised in non-NHS Wales laboratories.

The location of tests has been re-categorised from 12 August 2020 for current and historic data to reflect the different testing pathways an individual may experience. Samples that have been collected in the community from community testing units, mobile testing unit and mass testing centres where there is a clinical need have been combined into the community and mass testing in person category These have been identified using the asymptomatic/symptomatic field from WLIMS which was included in the Electronic Testing Record (ETR) from 18 June. This is primarily individuals who have not been identified as asymptomatic. This also includes hospital tests for asymptomatic individuals as part of pre-operative, pre-partum and oncology-related asymptomatic testing (which are generally completed on demand).

Samples collected from asymptomatic key workers and residents where critical workers are employed have been included in the satellite asymptomatic screening of key workers category. These have been identified as key workers and residents where critical workers are employed who have been recorded as asymptomatic on the ETR and are deemed to be on the asymptomatic keyworker screening pathway. The majority of these tests will take place in a setting outside of a test centre, such as a care home or prison and reflect a different process to that of samples collected in test centres.

Although the term satellite is used, some of the screening tests of asymptomatic keyworkers or residents included in this category may be conducted in person as part of wider mass and community testing.  Further work is needed to fully understand this distinction and may lead to some reallocation of tests between categories in future releases.

The asymptomatic aspect of the data has been collected from 18 June and included in this release from 22 June, so there is a full week of data. Prior to this the data was not available, therefore it is not possible to provide the breakdown. If the field is left blank in the ETR then it is assumed the individual is not asymptomatic and part of a key worker screening programme.

Hospitals

• All samples collected in hospitals in Wales are processed in NHS Wales laboratories and are reported in tables 7 and 8.

Community and mass testing in person

• Samples collected from community testing units, mobile testing units and mass testing centres processed in NHS Wales laboratories where there is a clinical need to be tested such as the individual displaying symptoms or preoperative screening tests.
• Community Testing Units in Wales are linked to health care settings and are operated by the local health boards. Testing is prioritised for health care workers but some do facilitate testing for other critical workers as capacity allows. Testing is organised at a local level by the health boards.
• Mass testing centres are typically drive-through testing centres and samples collected here are processed mainly in non-NHS Wales labs. However, there are some centres which have their samples processed in NHS Wales labs.
• A list of centres can be found in the Testing for coronavirus guidance: Regional coronavirus testing facilities.
• Tests processed through NHS Wales laboratories and non-NHS Wales laboratories (Lighthouse Labs) have been identified separately and included in different tables.

Satellite asymptomatic screening of keyworkers

• Samples collected from individuals who are part of a key worker or resident where a key worker is employed asymptomatic screening programme.
• The majority of samples are collected from settings other than a test centre and are known as satellite tests, for example fortnightly testing of staff at care homes as part of Welsh Government’s care home testing policy.
• Some samples included in this category could be from community or mass testing centres.

• Key worker screening tests may take longer to complete as these individuals are asymptomatic in the main and there is an extended delivery process compared to samples collected at test centres.

Organisation portal

• All tests that have been booked through the organisation portal and processed in non-NHS Wales laboratories.
• The majority of these tests are currently samples taken at care homes but this portal will soon open to other organisations.

   

Home tests

• All home tests are processed in non-NHS Wales laboratories and are reported in tables 9 and 10.

Timeliness of results

The timeliness of results authorised is presented in the Welsh Government’s weekly summary of testing data in the location of tests table. It shown as the proportion of tests which had an authorised result within 1 calendar day, 2 calendar days and 3 calendar days of the sample being collected from the patient. Results are presented in calendar days as many tests processed at NHS Wales labs are missing information on the time the specimen was collected. Work is ongoing to improve the reliability of capturing this information.

Turnaround times are calculated from the date the sample was taken to the date the result is released into the Welsh Laboratory Information Management System (WLIMS) and is available to clinicians. Hospital patients will be informed by treating clinicians, while most patients sampled through other routes are informed by the Public Health Wales text messaging service, which is the same day as the authorisation date in most instances.

Data on the time taken to receive a COVID-19 test result is split by testing route, as this will have an impact on the time taken to complete the test processing. Home tests need to be posted and couriered to lab. Key worker screening tests may take longer to complete as these individuals are asymptomatic in the main and there is an extended delivery process compared to samples collected at test centres. For example, care homes may complete batches of tests across different staff shifts before returning the tests by courier to the laboratory to be processed.

Tests on generally symptomatic individuals at test centres or individuals being tested as part of pre-operative, pre-partum and oncology-related asymptomatic testing (i.e. clinical need) will have a different testing pathway reflected by the clinical need to be tested.

These varying methods and pathways can have an impact on the turnaround time of the test.

A flowchart is provided on the headline page showing the process for each testing route and how the turnaround times are measured titled COVID-19 testing turnaround time flowchart.

Antibody testing

The number of antibody tests is presented in Welsh Government’s weekly summary of testing data outlining the number of positive, negative and equivocal results. If your antibody test is termed as ‘equivocal’, it means that the result is inconclusive.

Antibody testing is currently only available to priority groups with testing prioritised for:

• a sample of the school staff who have worked in the education hubs providing support to children of key workers and vulnerable children during the pandemic
• healthcare workers, with priority given to those who have potentially been repeatedly exposed to the virus