Covid lateral flow tests 'catch nearly everyone' at risk of spreading infection, study finds

Covid lateral flow tests "catch nearly everyone" at risk of spreading infection, a peer-reviewed study has found, as it raised concerns that past criticisms of the tests had "confused policymaking". 

Researchers said lateral flows were likely to be more than 80 per cent effective at detecting any level of Covid-19 infection in someone.

This rises to more than 90% at detecting those who are most infectious when using the test.  

Past criticisms of lateral flows (LFTs) for their apparent low sensitivity had been caused by wrongly comparing their results with PCR tests "like for like" when in fact "this is like comparing apples and oranges".

'Damaged public trust'

Previous research raising doubts about their effectiveness was "potentially misleading" because the tests worth in "a very different way", according to the report by UCL, Liverpool University, Harvard University and the University of Bath.

The study concluded that past studies had "reached the wrong conclusions" and "damaged public trust" when in fact lateral flow tests "are a reliable public health tool in stopping the spread of the virus".

Professor Irene Petersen of UCL's Institute of Epidemiology and Health Care, said: "Previous studies comparing the reliability of lateral flow tests and PCR tests could be potentially misleading because a PCR test is a marker of having been infected at some point within a certain window of time and does not necessarily mean someone is infectious when testing positive.

"In most validation studies, individuals were tested simultaneously with LFTs and PCR tests, with PCRs being used as a gold standard to say someone is 'positive or negative'. 

Negative lateral flow but positive PCR explained

"The sensitivity of the LFTs was therefore evaluated by their ability to identify the same cases that the PCRs picked up. 

"However, this is like comparing apples and oranges."

Researchers pointed out that LFTs detect material from the surface proteins of the virus and are very likely to give a positive result when someone is infectious.

Meanwhile, PCR tests detect the virus' genetic material, which can be present for weeks after a person is no longer infectious.

Professor Michael Mina of Harvard School of Public Health added: "It is most likely that if someone's LFT is negative but their PCR is positive then this is because they are not at peak transmissible stage."

The authors acknowledge that the sensitivity of the lateral flows is dependent on sampling errors and experience of the person performing the sampling and the test.

Professor Petersen added: "As LFTs are becoming widely used in schools, workplaces and for admittance to venues such as those used for large events, it is important that health professionals and the public have clear information about the operating characteristics of the tests. 

"We have demonstrated that the absolute sensitivity to detect SARS-CoV-2 antigens is likely high with LFTs."