October 12, 2021

expert reaction to media reports about some children having positive lateral flow tests but negative PCR tests

There have been several media reports about an increased number of school children receiving false positive lateral flow tests.

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

“The original dual-testing for approximately 6,000 of Liverpool’s asymptomatic citizens revealed 33 INNOVA LFT-positives, of whom two were void on PCR and 3/31 were PCR-negative (10%) but with wide 95% confidence interval from 2% to 28%, see https://www.liverpool.ac.uk/media/livacuk/research/Liverpool,Community,Testing,Pilot,Interim,Evaluation.pdf.

“Hence, I congratulate journalists for making use of the corroborative test-data that are held by NHS Test & Trace: they are, of course, consistent with the uncertainty from Liverpool but more importantly allow greater precision by dint of substantially larger numbers of LFT-positives, notably pupils, that sought PCR-corroboration.  The 3^rd-week September rate (as reported in the Times article) was 6.8% (957/14,000; 95% CI: 6.4% to 7.3%) whereas the 4^th-week rate was reportedly 12.5% (2000/16,000; 95% CI: 12.0% to 13.0%).  Clearly, something has changed: the question is what has changed – the rate, location or type of pupil who seeks PCR-confirmation of LFT-positive or cross-reaction with co-circulating virus or PCR-laboratory?  Questions, I’m afraid, not answers – other than that an answer is required.”

Dr James Gill, Honorary Clinical Lecturer, Warwick Medical School, said:

“Test and Trace, as has been recently commented on in the MP’s report into the pandemic handling, has been an area where things could have been managed more effectively.  We may now be seeing a further issue arising from Test and Trace with regard to communication over lateral flow tests.

“The original function of the COVID lateral flow test was as a screening tool.  The purpose of a screening tool is test apparently health individuals BEFORE symptoms develops, with the aim of starting treatment early – or in the case of covid instigating isolation as soon as possible.

“The lateral flow is not a tool for use with patient presenting with symptoms.  Anyone experiencing covid symptoms, dry cough, temp, lack of smell, should organise for a PCR test, regardless of a lateral flow test reading.  In general practice patients are surprised when we explain that whilst they might have a negative lateral flow test, they STILL need to have their symptoms checked with a PCR test.

“However, a lateral flow test showing positive result, and yet being shown to be a false positive via PCR, is showing how the system is supposed to work.  The lateral flow test is a SPECIFIC test, to correctly identifying people WITHOUT covid19 infection, but does not have the SENSITIVITY of the PCR test, which is used to detect people WITH covid19 infections.

“Understanding that subtle difference between sensitivity and specificity is always a large hurdle when we teach at medical school, so it is perhaps unreasonable to expect patients at home to grasp this difference, hence the need to greater clarity from Test and Trace in the public health messaging.”

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

www.sciencemediacentre.org/tag/covid-19

Eg. https://inews.co.uk/news/covid-lateral-flow-pcr-tests-positive-negative-results-school-parents-chaos-1243056

https://www.telegraph.co.uk/news/2021/10/11/false-positive-lateral-flow-tests-children-double-week/

Declared interests

Dr James Gill: “No interests to declare.”

None others received.