PCR v LFT: Lateral flows will speed up Covid isolation but could leave one in 10 with false positive

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The move to scrap confirmatory PCR checks for people who test positive with lateral flow has once again reignited the debate about which test is best.

Undoubtedly, allowing people to self-isolate from the first day they test positive by lateral flow will cut down the time spent removed from society as they await their PCR result, but it may also lead to others being forced to stay inside who would have been given the all clear.

Latest NHS Test and Trace figures from mid-December show that one in 10 lateral flow tests came back negative once checked by PCR. Under the new guidance, all those people would still be forced to self-isolate.

Experts argue that higher prevalence with omicron will mean fewer lateral flow positive tests are wrong.

But a Cochrane Review from last year suggested that at five per cent prevalence – not too dissimilar to the current 1 in 15 cases in England estimated by the Office for National Statistics (ONS) – we would still expect between 10 per cent and six per cent of symptomatic lateral flow positives to be false.

Dr Jac Dinnes, a senior test evaluation methodologist, from the University of Birmingham and co-author of the Cochrane Review, said: “With confirmatory PCR those people could be back to work within a couple of days, depending on testing capacity. Without it they will have to self-isolate according to whatever guideline is in place at the time.”

So what is the difference between lateral flow and PCR, and which should we trust? Here are the pros and cons of each.

Lateral flow

Lateral flow, also known as a rapid antigen test, uses mucus from the nose or throat which is diluted and placed on a porous strip.

The sample is pulled along the strip by capillary action where it meets a line of antibodies designed to recognise and bind to Covid-19. When the binding occurs, the line changes colour indicating a positive test.

The benefit lies in the speed, giving a result in under 30 minutes, rather than waiting up to 72 hours for the results of a PCR.

They are also useful at a population level for finding “hidden” carriers who may be transmitting the virus without knowing because they show no symptoms – an estimated one in three cases.

However, unlike PCRs they do not amplify the sample so their ability to detect the virus is far lower. It means that while they adequately pick up people while they are at their most infectious, they miss cases where the disease is rising or falling.

They also tend to throw up substantial numbers of false negatives, with around 28 per cent of symptomatic people wrongly told they do not have the virus and 42 per cent of asymptomatic carriers.

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