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Covid-19: Safety of lateral flow tests questioned after they are found to miss half of cases

BMJ 2020; 371 doi: https://doi.org/10.1136/bmj.m4744 (Published 04 December 2020) Cite this as: BMJ 2020;371:m4744

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False positives in Liverpool lateral flow test

Dear Editor

Criticism of the Liverpool lateral flow testing pilot has focused on “false negatives”, but “false positives” are also a problem. From the available data, one in two of the positive results by LFT in Liverpool will be “false positives”. People with a positive test result are instructed to self-isolate and to get a confirmatory PCR, but the latter is omitted from the text message they receive from NHS Test & Trace. [1] Neither the proportion who then get a PCR test, nor the proportion confirmed by PCR, are publicly available.

The scale of the “false positive” problem is apparent from the Government data on sensitivity (58%) and specificity (99.68%), together with the number of Liverpool residents tested by LFT (119,456 as of 2 Dec) and the number with positive result (798). From these figures, the prevalence in the relevant population – those Liverpool residents who choose to take an LFT test – is 0.6% and the positive predictive value, in this population, is 52%.

Neither the Liverpool City Council FAQs [1] nor the Government’s Annex B [2] acknowledge the scale of the problem. The FAQs simply state “there is a possibility that some people who test positive with LFT are 'false positive' and the gold standard PCR test will identify these.” Annex B says “The virus prevalence is around 1% in the country, so we expect around 7 true positives and 4 false positives for every 1000 people tested.”

The national value, including both symptomatic and asymptomatic cases, is not the relevant prevalence, but it suggests that the Government may also envisage using LFT on symptomatic cases, a possibility which is not excluded in the FAQs.

Greg Dropkin
Liverpool

[1] https://liverpool.gov.uk/communities-and-safety/emergency-planning/coron...

[2] https://www.gov.uk/government/publications/community-testing-explainer/c...

Competing interests: No competing interests

06 December 2020
Greg Dropkin
retired NHS administration worker
Liverpool L8