Lateral Flow Tests Detect Vast Majority of People At Risk of Transmitting COVID-19

Dawn O'Shea

October 14, 2021

A new study led by researchers at University College London (UCL) has found that lateral flow tests (LFTs) are more accurate at detecting COVID-19 infection than previously reported.

Previous research has compared the efficacy of LFTs to the gold standard PCR tests, despite differences in the method of both tests. While PCR tests detect the virus’ genetic material, which can be present for weeks after the acute infection, LFTs detect material from the surface proteins of the virus and can detect SARS-CoV-2 only when the virus is present at high levels. PCR is capable of detecting levels of virus that are too low to be associated with an appreciable probability of infection. Consequently, comparing the two directly is akin to comparing apples and oranges.

In this new study, published today in Clinical Epidemiology, the researchers from UCL, the University of Liverpool, Harvard University and the University of Bath use a new formula to determine the accuracy of LFTs that takes into account the differences between LFTs and PCR tests.

The formula shows that LFTs are likely to be more than 80 per cent effective at detecting any level of COVID-19 infection and more than 90 per cent effective at detecting the virus in those who are most infectious.

The formula was tested using data from a previously-reported head-to-head study which suggested that LFTs had a sensitivity of just 40 per cent compared to PCR tests. However, after applying the formula, which takes into account the differences between the tests and the biology of COVID-19, the UCL-led team suggests that, in reality, the sensitivity of a typical LFT in identifying people who are infectious is greater than 80 per cent. The tests could achieve even 100 per cent sensitivity when viral loads are at their peak and therefore catch almost everyone who is currently infectious, they say. If someone tests negative on LFT but positive on PCR, this is less likely to be due to sensitivity and more likely to mean that the individual is not at peak transmissible stage.

Lead author, Professor Irene Petersen, from the UCL Institute of Epidemiology and Health Care, explained: “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’. 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.”

The authors conclude that LFTs are a reliable public health tool in stopping the spread of the virus.

“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,” Professor Petersen said.

She added that longitudinal studies where individuals and case contacts are tested daily by LFTs and PCR tests would help to further understand false negatives and false positives and, importantly, the time differences between PCR positivity, LFT positivity, and symptom onset.

Dr Hayley Jones, Senior Lecturer in Medical Statistics at the University of Bristol, cautioned that LFTs are not fool-proof. “Even if the true sensitivity of lateral flow tests to detect infectiousness were as high as the authors claim it could be (possibly “above 80%” according this paper), a negative lateral flow test result isn’t sufficient to rule out carrying the virus or being currently infectious, especially during periods when there is a lot of SARS-CoV-2 in circulation. 

“People should be careful not to treat a negative lateral flow test result as a definitive “green light”, especially if they have symptoms,” she warned.

This article originally appeared on Univadis, part of the Medscape Professional Network.

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