Covid-19 testing: how effective are lateral flow tests?

Covid-19 testing for New Year's Eve 2021: how effective and accurate are lateral flow tests taken at home?

It feels like everyone we know has been frantically taking lateral flow tests, making it harder to order tests on the NHS website and also, more of a stressful process to check if you have coronavirus before spending time with family and friends this New Year. But just how effective are the home lateral flow tests for identifying Covid-19?

In the week before Christmas, when we’d expected to be out last minute shopping and ringing in the festive season with loved ones, many of us stayed at home, kept ourselves safe and awaited coronavirus test results amid the surging Omicron variant

On Monday 27th December, the UK reported 109,077 cases of Covid. With rising cases, and the NHS website regularly reporting no available rapid antigen Covid tests, aka lateral flow tests, questions are being raised about their accuracy.

In mid-December, an Instagram post by Nathan, a London-based A&E doctor, went viral, showing the four lateral flow tests that he’d taken on the same day. Two came back positive and two had shown a negative result.

“LFTs are >99% specific for Covid,” he wrote in the caption. “That means if I don’t have Covid, there’s < 1% chance of it being a false positive.”

He went on to explain: “However, LFTs are only ~ 58% sensitive. (i.e if I do have asymptomatic COVID, a LFT will only detect it 58% of the time). This is demonstrated beautifully here by having two negative tests and two positive tests all taken the same day).”

“I now need to self isolate and get a PCR as soon as possible,” he added, referencing the polymerase chain reaction (PCR) tests, which are considered to be the most accurate form of coronavirus testing.

Indeed, with group chats and social media timelines awash with people receiving “false” negative lateral flow results, how effective is lateral flow testing against Covid-19? Stylist spoke to Dr Punam Krishan, an NHS GP, to find out.

Lateral flow tests vs. PCRs

“Lateral flow tests should be used on a regular basis and especially if you’re going out to meet others if you are well and have no symptoms,” Dr Krishan tells Stylist. “Too many people I see and hear have symptoms suggestive of Covid, but are doing LFTs and, if negative, are going about their daily business only to discover a few days later on PCR that they have indeed been positive all along.”

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How do lateral flow tests work?

“The LFTs detect the protein of the virus. Before you actually become infective enough to pass it on to others the virus silently grows and multiplies until your immune system mounts a response strong enough to attack it,” Dr Krishan explains. “The LFTs are effective during this period and can help you identify the virus before you become symptomatic. However, once the immune system starts attacking the virus, it causes the release of the virus’ genetic material into the system. This is when symptoms usually begin.

“This genetic material cannot be detected by the LFT and this is why we advise everyone with symptoms to do a PCR test, which is far more sensitive and specific to detecting Covid, as well as being able to help identify which variant it could be, which LFTs cannot do.”

How effective are lateral flow tests?

Studies suggest that, more often than not, lateral flow tests return an accurate result.

A review of 64 studies found that they correctly identify 72% of infected people who have symptoms, and 78% within the first week of becoming ill. Furthermore, the Department of Health and Social Care claims that the Innova LFT detects more than 95% of individuals with highest ‘viral loads’ – those who are most likely to spread the virus.

However, as Dr Krishan points out, there are a few reasons why a negative lateral flow test could be wrong. “You might not be swabbing thoroughly enough, or could have a faulty test kit,” she suggests. However, she stresses, if ever a doubt goes through your mind that you might have Covid, get a PCR.

“The other thing is that after recovery from Covid it can take up to 90 days for the virus to completely be mopped up from the body. As a result, for many people the PCR test may continue to show positive during this time,” she adds.

“It is important to follow all the advice given because it is possible to be reinfected with Covid even if you have had vaccines and Covid.”

“I know people find it very frustrating and restricting at the moment but it is important that if you develop any of the symptoms that you promptly self isolate and do a PCR test because the current strain of Omicron is extremely infectious and is transmitting four times faster than the Delta variant,” Dr Krishan shares. “I urge everyone to get their vaccine and their booster. Remember that vaccines, in conjunction with regular testing, hand washing, wearing face coverings, social distancing and ventilating are the best tools we’ve got to keep ourselves and each other safe.”

For more information about testing, vaccines and Covid-19 advice, visit the Public Health England or NHS websites.

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