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expert reaction to news from UKSHA that COVID-19 testing at a private lab has been suspended after NHS Test and Trace investigation

The UK Health Security Agency (UKHSA) has announced that testing at a private lab has been suspended following reports of people receiving negative PCR test results after they have tested positive on a Lateral Flow Device.


On the question of how many new infections may have resulted from this error: Prof Paul Hunter, Professor in Medicine, The Norwich School of Medicine, University of East Anglia, said:

“This is difficult to answer with certainty.

“R is about 1.0 now so on average each case has been leading to one new infection.

“But people are most infectious from 2 days before to 3 days after becoming ill see:

“If the LFD test was taken for the onset of symptoms and was positive the waiting a couple of days to get the PCR back then by the time the false negative PCR test came back most of the risk of transmission would be past

“But if taken before symptoms then by the time the false negative PCR test is back could be still 1 or 2 out of the 5 most infectious days.

“Of course this assumes that people actually obey the guidance on self-isolating which many do not.

“If half of the positive LFD tests were done because of symptoms, taking all this together I would guess that the rate of transmission events  that would have been prevented if the PCR test had been correct would be of the order of 10 to 20% or about 4 to 8K. However, this is probably lower because of lack of compliance with self-isolation even in people known to be positive.

“But these numbers are very rough and should be taken more as an indication of the factors influencing preventable transmission than an exact value.”


Dr Simon Clarke, Associate Professor in Cellular Microbiology at the University of Reading, said:

“The news that a COVID-19 testing laboratory has potentially returned 43,000 false negative results is alarming.  While we don’t know where the fault occurred, there must have been error somewhere, otherwise this usually reliable test would not have failed to this extent.

“This comes on the back of a great deal of speculation, particularly coming from South West England, as to why so many people were getting positive results from lateral flow tests, but negative results from the follow-up PCR tests.  It now seems that in many cases, it was a failure in the PCR test, rather than in the lateral flow devices, or indeed a new variant.

“While it’s true to say that most of the people mis-diagnosed as not having COVID-19 are no longer infectious, it is somewhat complacent as most of these people will have carried on with their normal lives and spread the infection further.  These missed opportunities to cut thousands of chains of infections mean that thousands more people are infected today.

“Given that the government has effectively abandoned a whole range of measures to prevent spread, such as social distancing, mask wearing, and restrictions on social mixing, it is essential that the few tools we have left, including accurate tests, work properly. If people lose confidence in the accuracy of the so-called “gold standard” PCR tests, they are less likely to believe or act on positive test results. This would further dent our ability to prevent the spread of covid, and put vulnerable people at greater risk.”


Dr Alexander Edwards, Associate Professor in Biomedical Technology, Reading School of Pharmacy, University of Reading, said:

“While this news is very disappointing- especially for those people affected – we should remember that the sheer scale of covid-19 testing is unprecedented. Hopefully now this particular problem has been identified: the quality processes that should be built into any clinical diagnostics system will be able to fix the specific problem and rapidly contact everyone affected.

“This announcement is very helpful in making sense of repeated reports of people receiving conflicting results. Transparency is critical.

“Let’s not throw the baby out with the bathwater. The majority of test results are correct, and it’s worth remembering that our testing system has been built up from almost nothing at the start of the pandemic. I am genuinely amazed by this expansion of testing, and I applaud all the hard work that has gone into this on so many levels. Many people dropped everything they were doing to focus fully on high-quality, scientifically sound testing, and following months of incredibly hard work have not received the same appreciation showered on vaccine workers. Please let’s not assume all problems or failures mean that all testing is bad.

Is it still possible that this happens without issues in a testing facility? i.e. if people have had a positive lateral flow test and then negative PCR but are not contacted, should they be reassured that they do not have COVID?

“It’s always possible to have positive followed by negative tests, statistically this is going to happen for a proportion of people, and the number of people in this position will vary over time.

“One big problem is that there is not really a reliable “rule-out” test for an infection like COVID. Humans are messy to test! This is why it’s hard to provide reassurance to individuals that they don’t have Covid-19. Testing is very useful to reduce risk on a population level, but in order to rule out a condition for an individual you need a very high test sensitivity. But the nature of testing for a respiratory virus infection makes it hard to achieve very high sensitivity. If, when taking a swab sample, virus is not collected, no test can detect infection. We know the amount of virus on a swab can vary enormously, making it a hard analytical challenge. The virus grows very rapidly, and so it’s always possible to test negative but be infected shortly afterwards. We also know people can remain testing positive long after they have recovered, making it hard to tell the difference between someone who has recovered from Covid-19, from someone just at the start of an infection, or someone with just a small amount of virus on a swab.

“There is plenty of evidence that both lateral flow tests and PCR very rarely suffer from false positives (although lateral flow tests can fail, for example if they are deliberately used incorrectly e.g. adding certain drinks instead of a real sample).

“Longer term, this type of problem can be tackled with more healthcare professionals, trained to understand testing, and more testing experts and test site workers who understand clinical diagnostics, to provide resilience for our country. Quality clinical diagnostics is a critical area of public safety.”


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

“Speed and method of detection at UKHSA/NHS T&T matter: that a problem existed whereby the proportion of LFT-positive tests that was PCR-negative has increased dramatically across England. The source of the problem turned out to be a specific private laboratory. Unclear as yet are the following:

Did the laboratory’s internal quality control fail to signal?

Did inter-laboratory quality-control fail to signal?

“Identification was via PCR-adjudication of LFT-positive tests (and journalists’ reporting): however, does the “problem” affect PCR tests done for persons without LFT-positive antecedent, who include citizens who have developed symptoms; who have been identified as close contacts of index infected persons and are following advice to seek immediate PCR; or are international travellers arriving into or back to UK?

“Hence, the further information from UKSHA should identify the implicated laboratory’s weekly share of the PCR-testing in England in a range of specific contexts such as those listed above; together with its results per-context per week (versus in comparable other laboratories) so that trigger-points for when/if the problem affected different PCR-contexts can be discerned.”



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



Declared interests

Dr Alexander Edwards: “No conflicts.”

Prof Sheila Bird: “SMB is a member of RSS Working Group on Diagnostic Tests which reported in June 2021, see

None others received.



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