Lateral Flow Tests (LFTs), being used widely in the UK in schools and workplaces to test COVID-19 transmission risk, are more accurate than previously reported and cannot be compared directly to how Polymerase Chain Reaction (PCR) tests work, a new study led by University College of London (UCL) researchers said on Thursday.

In a paper published in ‘Clinical Epidemiology’, the scientists note that LFTs are likely more than 80 percent effective at detecting any level of COVID-19 infection and likely over 90 percent effective at detecting those who are most infectious when using the test.

This level of accuracy is much higher than some previous studies have suggested, and the authors say the tests are a reliable public health tool in stopping the spread of the infectious virus.

“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,” said Professor Irene Petersen of the UCL Institute of Epidemiology and Health Care and lead author of the paper.

“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’. 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,” he said.

The researchers from UCL, Liverpool University, Harvard University, and the University of Bath highlight that LFTs work in a very different way to PCR tests and cannot be compared “like for like”.

LFTs detect material from the surface proteins of the virus and are very likely to give a positive result when someone is infectious whereas PCR tests detect the virus’ genetic material, which can be present for weeks after a person is no longer infectious.

Professor Petersen said: “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 (COVID-19) antigens is likely high with LFTs.

“To improve our understanding of their characteristics, longitudinal studies where individuals, and ideally contacts of cases, are tested daily by LFTs and PCR tests would help to further understand false negatives (and false positives) and, importantly, the time differences of between turning PCR positive, LFT positive, and symptom onset,” Professor Petersen said.

The paper presents a formula for calibrating the sensitivity of LFTs using an illustrative example from a study in Liverpool.

In the Liverpool study, a head-to-head validation suggested the sensitivity of LFT was only 40 percent. However, after taking into account the differences between the tests and the biology of COVID-19 the UCL-led team suggests that, in reality, the sensitivity of the typical LFT in being able to identify someone who is likely to be infectious, is above 80 percent.

“There is a spectrum of infectious amounts of the COVID-19 virus and we show that LFTs are likely to detect cases 90-95 percent of the time when people are at their most infectious,” said Professor Michael Mina of the Harvard School of Public Health.

“The tests could achieve even 100 percent sensitivity when viral loads are at their peak and therefore catch nearly everyone who is currently a serious risk to public health. 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,” he added. 

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