#16,246
In the UK, the home use of LFTs (Lateral Flow Tests) to rapidly detect COVID is far more common than in the United States, in large part because their government subsidizes the costs. The UK NHS testing website suggests that persons without COVID symptoms test regularly.
Why you should get tested regularlyWhile not as precise as a PCR test, these home screening kits offer reasonable sensitivity (ability to detect the virus), and generally very good specificity (ability to rule out infection). In other words, it may not always detect the virus, but it rarely produces a false positive.
About 1 in 3 people with COVID-19 do not have symptoms but can still infect others.
You should do a rapid test twice a week (every 3 to 4 days) to check if you have the virus. If people test positive and self-isolate, it helps stop the virus spreading.
Even if you’re vaccinated, there’s still a chance you can pass COVID-19 on, so you should keep getting tested regularly.
Anyone who tests positive with the home LFT is then urged to isolate, and get a confirmatory PCR test.
Over the past few weeks we've seen increasing reports of non-COVID respiratory infections in the UK (see `Worst Cold Ever' Trending On Social Media). In recent days, social media and the UK mainstream media are reporting that many of these cases - particularly in Southwest England - are testing `positive' with the home LFT, but their PCR tests are coming back negative.
New Covid variant fears erupt over mysterious test results - urgent investigation launched - MSN.Com
Calls for inquiry as negative Covid PCR tests after positive lateral flow reported - The Guardian
Lateral flow tests: Positive rapid Covid test results followed by negative PCRs causing chaos in schools - Inews
Granted, in February of 2021 we saw a flurry of reports of a COVID variant in Finland (see What (Little) We Currently Know About the Fin-796H Variant) that could elude standard PCR testing. And the inability of one brand of PCR test (Thermo Fisher Scientific Taq Path) to detect B.1.1.7 actually helped in the identification of the Alpha variant.
So a new variant isn't completely out of the question. But for now, this is a testing anomaly. and the most obvious suspects are either the home LFT kits - or possibly the laboratory PCR tests - being used in Southwest England.
The UK Health Security Agency (UKHSA) is reportedly investigating, although they have yet to publish anything on their website or on their twitter feed @UKHSA. Right now, it is hard to say whether this amounts to anything, or is simply a tempest in an LFT-pot.
Hopefully we'll have some clarity soon.
In the meantime, we have some expert opinions offered up this morning on the Science Media Centre website.
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 3rd-week September rate (as reported in the Times article) was 6.8% (957/14,000; 95% CI: 6.4% to 7.3%) whereas the 4th-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.”
