Your complete guide to lateral flow tests – and whether they might save your Christmas

For the past few months, our lives have been ruled by a swab up the nose. Comparisons to a pregnancy test (which the lateral flow resembles) are apt: there’s a breathless wait for the line to appear – but just the one, not the two, please! – because only negative results are welcomed.

A negative lateral flow test has meant our kids can go to school, we can take a holiday, or visit certain cultural events. But in these last few days before Christmas, the LFT has taken on a greater import. For many, a solitary red line now dictates whether we can celebrate the season at all.

Ever since Professor Chris Whitty told us to “prioritise the social interactions that mean a lot to you,” we’ve been in a mental maelstrom as to how to behave. To avoid infection and a positive test, some people are cancelling get-togethers, taking pre-Christmas time off or working from home. And in the absence of further Government restrictions (for now...), LFTs have become something of an arbiter. Many of us are self-regulating, using them as a passport to party or visit our parents – even if we don’t have any symptoms of ill-health.

Stephen Reicher, professor of psychology at the University of St Andrews and a member of the Scientific Pandemic Insights Group, this week advised: “If you want a good Christmas dinner, I would say be very careful about meeting up before Christmas. But you can do things to stack the odds in your favour if you ever do meet up – the first thing is to make sure you have a lateral flow test.”

Here’s everything you need to know about LFTs:

Remind me: what are they?

Otherwise known as a rapid test, an LFT detects proteins from the Covid virus, which are present when someone is infectious. An LFT contains a strip of antibodies which turns red if it interacts with a protein from the virus’s shell. It’s free, can be used at home, and gives results in minutes. The more accurate PCR (polymerase chain reaction) test has to be sent to a laboratory.

Can I still get one?

Yes – they are still free from your local pharmacy, or collection point, shown here https://maps.test-and-trace.nhs.uk. In the pre-Christmas rush, customers have been reporting shortages in some areas. But according to The UK Health Security Agency, more than 11 million test kits were sent out to pharmacies last week – double the number in the previous week. Most people should find a test locally, they insist.

Are LFTs accurate?

Early studies put the sensitivity of LFTs at a not-too reassuring 40 to 60 per cent. But a recent study by UCL epidemiologist Professor Irene Peterson argued that PCR tests were arguably too sensitive. “Previous studies... could potentially be misleading because a PCR is a marker of having been infected at some point within a certain window, and does not necessarily mean someone is infectious when being tested,” she says.

Peterson suggested the likely sensitivity to be 80 per cent, whereas Harvard Professor Dr Michael Mina said LFTs “were likely to detect cases 90 to 95 per cent of the time when people are at their most infectious.”

What if I only get a faint second red line?

In the same way you can’t be a ‘bit’ pregnant, you can’t only be ‘bit’ positive for Covid. “I’m currently worried to see lots of people reading a faint line next to the T on an LFT as an inconclusive result,” says Dr Hayley Jones, senior lecturer at the Bristol Medical School clinical epidemiology unit. “A faint line on these tests is a positive result – and very often your first positive LFT will give only a faint line. So do be careful to check the device closely, isolate and get a PCR test if you see any band. As the amount of Covid in the population is currently so high, the chance of your weak positive LFT result being correct rather than a false positive is also currently very high.”

Do LFTs work as well on the omicron variant?

It seems so. Although omicron has around 50 mutations, the antibodies in LFTs still bind to the proteins of the coronavirus. “Initial laboratory validation has determined similar sensitivity to detect omicron compared to delta,” said the UK Health Security Agency last week. But there is still some chance of false negatives.

If I have symptoms, should I go straight to a PCR? Even if my lateral flow is negative?

“The Government guidance on this remains unclear and confusing,” says Jones. “You’re told to get a PCR test if you have one of the three ‘classic’ symptoms, (fever, cough, or loss of sense of smell or taste), and take an LFT if you have no symptoms. This doesn’t tell you what to do if you have a runny nose, sore throat etc – and most people with Covid are now first reporting symptoms more like a classic cold. This guidance desperately needs to be updated... if I had mild symptoms and a negative LFT, I would get a PCR test.”

Are LFTs ‘good enough’ before we set off to see elderly parents?

According to Jones. this is an individual decision and will depend heavily on context. “A negative LFT result, especially if immediately before the contact, means you’re less likely to be infectious at that point in time,” she says. “Your decision on whether this reassures you will depend on a number of other factors, such as whether your elderly relatives are vaccinated, whether they are particularly vulnerable, your own recent levels of social contact (particularly any known contact with someone with Covid) and, obviously, whether you have any symptoms.

“We talk of pre-test and post-test likelihood of having Covid,” she says. “If you think there’s a high chance that you are infected, then you should not let a negative LFT alone change your mind, and consider a PCR.”