They have become an integral part of millions of people’s lives: lateral flow tests (LFTs) are taken daily by some for work or socialising, while new guidance means that anyone with Covid will be allowed to finish their isolation period after seven days rather than 10 if they have two negative results.
Here’s all you need to know about the rapid tests, from what experts have to say on their reliability to why you may repeatedly test positive many days after an initial Covid infection.
How do LFTs work and how reliable are they, including for Omicron?
LFTs identify specific viral proteins that are present inside your nose when coronavirus is infecting you.
If you have come into contact with an infected person, it could take a few days for a positive result to show up. “Please continue to test on a daily basis,” urges Irene Petersen, professor of epidemiology and health informatics at University College London.
The reliability of LFTs depends in part on whether you follow the instructions carefully, which is why studies give varying estimates of their efficacy, from 40% to 90%. A recent paper demonstrated that LFTs, when used correctly, are likely to have a sensitivity above 80% and in many cases above 90%. After carrying out tests, the UK Health Security Agency said LFTs have a “comparable sensitivity” for Omicron as for other Covid variants such as Delta.
The main benefit of LFTs is they can be carried out quickly and easily at home, with a result in under 30 minutes, and that they don’t need to be sent to a lab. However, the gold standard remains the polymerase chain reaction (PCR) test, which detects viral load and infectiousness at lower levels, including before and after the time range in which they are picked up by LFTs.
How long before meeting family and friends should you take one?
You could become infectious after you take the test, which is why scientists recommend taking it as close to your social plans as possible, ideally within an hour. “Omicron replicates very fast, so you may become very infectious within hours,” says Petersen.
If you took one early in the morning and are going out in the evening, you might take another, provided there has been a gap of six to 12 hours. However, it’s worth bearing in mind that there is a test shortage at the moment, and they are urgently required for key workers, so use them strategically.
Petersen emphasises that “no test offers 100% accuracy, so please consider the risk of getting severe illness among your friends and family”.
Does the thickness of the line on an LFT tell you how infectious you are?
Double lines appearing on an LFT – however faint – mean you have detectable virus in your nose and are therefore infectious. Azeem Majid, a professor of primary care and public health at Imperial College London, notes that the test is “not designed to estimate how infectious you may be”. However, David Matthews, a professor of virology at the University of Bristol, says that a thicker line could mean – in theory at least – there is more N protein. “That should mean more virus and therefore more infectivity in principle, but no one has tested that idea,” he says.
If you are testing positive on an LFT at the end of the isolation period, are you still infectious and do you still need to isolate?
NHS rules state that your isolation begins on the day that your symptoms develop or the day that you receive a positive result if you don’t have symptoms, and ends 10 days later. You can cut this isolation period down to seven days if you receive negative LFTs on days six and seven, 24 hours apart. The new guidance applies to everyone who has tested positive, regardless of vaccination status. (A different rule applies if you are an unvaccinated person who has been in close contact with someone who has tested positive: you must self-isolate for 10 days.)
Majid says there is no need to keep on testing repeatedly if you have had a positive LFT result confirmed by a PCR test. “Test results can sometimes remain positive for a period after your isolation ends, but this does not necessarily mean you are still infectious.”
Petersen agrees that people are “unlikely” to remain infectious, though it is not impossible. It’s more likely that “there are some LFTs which seem to be able to detect a very small amount of viral protein for a long time”, she says.
Matthews suggests getting a PCR test, and if that is also positive you could seek additional medical advice, as, although unlikely, in a very small number of cases it may mean you have an underlying medical condition.
How does the seven-day isolation rule square with earlier guidance advising against taking LFTs for 90 days after a positive PCR test unless you exhibit new symptoms?
This original guidance reflects the low rate of reinfection during this window, meaning false positives could force people to unnecessarily self-isolate. However, complete data is not yet available for Omicron reinfection rates, so it could change.
Matthews says the new guidance does not negate the earlier advice. Instead, “What they are saying now is that if you are LFT-negative on two days then you are very likely no longer contagious and can go to work”.
There is some suggestion that Omicron is taking an especially long time to show up on LFTs. What does this mean for how people should interpret LFTs?
Petersen says there are two probable reasons for this. First, it’s likely that vaccinated people or those with immunity from prior infection have symptoms even before they become infectious, as these are immune responses. Second, it is possible – though uncertain – that transmission with Omicron occurs at lower viral loads. “If the latter is the case, it is important that people start to isolate as soon as they have any symptoms that suggest they may have Covid, even if the LFT is negative,” she says.
If you’ve tested positive, including for Omicron, does that mean you won’t get Covid again any time soon?
Reinfections can occur after a previous Covid-19 infection. This is especially common with Omicron, which according to a recent Imperial College London report is more than five times more likely to cause reinfection than the Delta variant. These reinfections are most likely to have been in those who were previously infected with an earlier variant, such as Delta or Alpha.
This means you should continue to be careful, but it isn’t cause for excessive concern. Matthews says that any reinfection will “probably be a mild cold” as you will have built up “a bigger and better army of ‘memory’ T and B cells”, as long as you are otherwise healthy.
