April 15, 2021

expert reaction to media report suggesting ‘82% of positive rapid tests ‘were correct’’, as reported by the BBC

In an article by the BBC, it’s suggested that analysis of data from Public Health England (PHE) shows 82% of positive rapid tests sent off for a confirmatory PCR test returned the same positive result.

Prof Irene Petersen, Professor of Epidemiology and Health Informatics, UCL, said:

“It is good to see data reportedly from Public Health England are now coming through on the accuracy of the lateral flow tests performed in England during March/April 2021.

“According to the media report, most of these tests were performed in educational setting (81%).  The results illustrate that the tests work well in terms of covid-19 ‘detectors’.  Thus, more than 30,000 positive cases were identified within the first month of the mass testing program.  It is unlikely, that many of these individuals would have been identified otherwise.

“There has been some concern about testing on large scale when the prevalence of covid-19 in most communities are low.  Some researchers have suggested that the vast majority of the cases might be false positive.  However, it turned out to be a storm in a teacup.  With the most recent data, the current estimates suggest that the false positive rate is likely to be below 0.02%.

“The data provided on the BBC website suggest that out of 16,952 positive lateral flow tests conducted in March/April 82% were confirmed with a PCR test.  In other words, the current positive predictive values (PPV) of lateral flow tests are much higher than initially anticipated.  Yet, it is good practice to confirm positive test results when the prevalence of covid-19 is low as this minimise the risk of false positives even further.”

Prof Sheila Bird, Formerly Programme Leader, MRC Biostatistics Unit, University of Cambridge, said:

“PCR-adjudication of secondary pupils’ LFT-positives on return to school in March has not yet been disclosed.

“On the TODAY programme and online by BBC’s health reporter, Jim Reed, new data were announced, reportedly from Public Health England, ahead of today’s release of official statistics by NHS Test and Trace.

“My surprise is because I’d been warned that long-awaited data on PCR-adjudication of positive Lateral Flow Tests taken by secondary pupils during 4 to 19 March 2021 was delayed in accordance with mourning for HRH The Prince Philip.

“Secondary school pupils positive lateral flow tests during 4 to 24 March numbered 6,075 (out of 9.45 million LFTs), half of them (3,012) in the week of 18 to 24 March.  Readers will recall that PCR-adjudication of secondary pupils’ positive LFTs during this period was discouraged: not least because the PCR-result would be ignored.

“The data disclosed to the BBC do not apply directly to secondary pupils.  How do I know?  Consider what has been reported:

1. Public Health England noted that secondary schools and colleges in England had been using LFTs since 8 March along with care home, universities and some workplaces.
2. 24,144,449 lateral flow test results were recorded by Public Health England from 8 March to 4 April (different weeks than NHS Test & Trace’s “weeks”).
3. 30,904 positive LFTs were found (1280 per 1 million LFTs), whereas the LFT positive rate for secondary pupils during 4 to 24 March was 6,075/9,449,610 (643 per 1 million LFTs).
4. Over half (55%) of the 30,904 positive LFTs were subject to PCR-confirmation. 3,034 (18%) were PCR-negative; 13,918 (82%) were PCR-positive.
5. Secondary pupils’ contribution to the 24 million LFTs in 4-weeks was not reported but may have been half.
6. Secondary pupils’ contribution to 30,094 LFT-positives in 4-weeks was not reported but may have been around 7,700 (643 * 12 million).
7. Secondary pupils’ contribution to 16,952 PCR-adjudications in 4-weeks was not reported but, having been discouraged, may be at best one-third of 7,700, say 2,500.

“And so we wait until next week to find out what % of secondary pupils’ PCR-adjudicated LFT-positives during 4 to 19 March were confirmed PCR-positives.

“Meantime, today’s data from Public Health England re-emphasise the importance of prompt PCR-confirmation of LFT-positive findings for three reasons:

1. a) one in six PCR-confirmations may release you and your contacts from self-isolation;
2. b) PCR-confirmation rate needs to increase substantially from 55% because PCR-testing is our route into monitoring genomic variation;
3. c) via PCR-tests, we scrutinize cycle-thresholds, as proxy for viral load. Cycle threshold is likely to be different in those who, despite vaccination, become infected.”

https://www.bbc.co.uk/news/health-56750460

All our previous output on this subject can be seen at this weblink:

www.sciencemediacentre.org/tag/covid-19

Declared interests

Prof Sheila Bird: “SMB serves on Royal Statistical Society’s COVID-19 Taskforce.”

Prof Irene Petersen: “I have no conflicts of interest.”