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expert reaction to the weekly statistics for NHS Test and Trace (England): 15 April to 21 April 2021, specifically around PCR-adjudication of secondary school pupils’ positive lateral flow test results

The government have released the latest statistics from the COVID-19 Test and Trace system.


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

“PCR-adjudication of secondary pupils’ ‘assisted’ (within schools) LFT-positives on return to school during the 4 to 19 March has at last been disclosed. But it has taken 6 weeks for the truth to out.

The truth is that under half of PCR-adjudicated LFT-positives were confirmed as PCR-positive. This was anticipated by the Royal Statistical Society’s COVID-19 Taskforce statement on 5 March 2021.

“Between the 4 to 19 March PCR-adjudication of secondary pupils’ positive LFT results was discouraged: not least because the PCR-result would be ignored. For the critical fortnight [4 to 17 March], when most positive LFTs from secondary pupils were monitored/assisted, parents’ common-sense ensured that at least 1,050 (45.6%) of the pupils’ 2,304 assisted LFT-positives were subject to PCR-adjudication. There were 1,033 non-void matched PCR-adjudications of which only 41% were PCR-positive (428/1,033; 95% CI: 38% to 44%). For detail, please see Table.

“For NHS Test & Trace to ignore PCR-adjudications was a public health own-goal and reminds us all that working at pace is no excuse for failing properly to set out prior assumptions (on prevalence of infection, in-context LFT sensitivity, LFT specificity) and hence work out the likely percentage of LFT-positives that are true positives. Failing to work through this meant failing to anticipate the likely poor consequences – ie the numbers of pupils, staff and family-members inappropriately quarantined – if PCR-adjudication was not strongly endorsed. The longer-term consequence is to have undermined the need for PCR-adjudication when we need it for assessing ct-thresholds (as proxy for viral load, which immunization reduces) and for discovery of new variants.

“The yellow section of the table below coincides with both the transition to SELF-REPORT home-testing and the Easter vacation. The LFT-positive-rate per 1 million tests has doubled but whether due to relaxation of precautions or due to under-reporting of LFT-negative home-tests or both is unclear. Moreover, when pupils’ LFTs were assisted, there was no doubt that a secondary pupil was the person tested whereas, home-tests – even if intended for secondary pupil – could be used by ANY member of their household.

“Comparison, for the same week, between the PCR-confirmation rate for LFT-positives between the upper (assisted) and lower (self-reported) panel of my table shows major disparity: the PCR-positive rate is substantially higher for home-test LFT-positives. In the week of 1-7 April, when LFTs were predominantly self-reported, the very high PCR-positive adjudication-rate of at least 80% seems altogether too high if, truly there were 1,000 infections per 1 million persons and LFT-false-positive rate were indeed as low as 3 per 10,000.

“Covariate information about those who self-report LFT-positive information, including how many home-LFTs were taken in the 2-weeks prior to the week of this positive-LFT and how many in the 6 days prior to positive LFT, may give some insight into how home-tests are deployed  – at least by those who test LFT-positive. PCR-confirmation rates need to increase as still below 60%.”



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



Declared interests

Prof Sheila Bird: “SMB served on UK’s Medicines Commission (1991-1995); supervised the doctoral thesis on pharmacovigilance in Uganda of Dr Ronald Kiguba; and gave Edinburgh University’s David Finney Centenary Lecture in 2017.”

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