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.”