By Clare Wilson
With the omicron variant of the coronavirus on the rise, regular testing has become even more important, particularly as people decide whether to socialise during the Christmas period. Here’s everything you need to know about testing.
What is the difference between a lateral flow test and a PCR test?
Polymerase chain reaction (PCR) tests were the first available for spotting the coronavirus. They work by detecting the virus’s genetic material, and are very accurate. Then came rapid tests, also called lateral flow tests (LFTs). Unlike PCR tests, these detect proteins from the virus, which are present when someone is infectious. Although both begin with someone taking a swab from their nose or throat, most PCR tests have to be sent to a laboratory, while rapid LFTs can be completed by the user at home and give results in minutes.
Are LFTs less good than PCRs?
Rapid tests aren’t as good as PCR tests at detecting the coronavirus because PCR tests involve a step where the genetic material is multiplied over and over again, so they can detect tiny starting amounts. LFTs have no multiplying stage, so may miss infections where virus levels are low.
Can we quantify the difference?
Various studies have put the sensitivity of LFTs – in other words, their ability to detect the virus if it is there – at about 40 to 60 per cent. That sounds unhelpfully low, but it is an unfair measure as it compares LFTs to PCR tests, which are arguably too sensitive, says Irene Petersen at University College London. In the weeks following an infection, the cells of the nose and throat can retain fragments of virus genetic material that aren’t infectious, but that can be amplified by the PCR process, leading to a positive result.
Taking this into account, modelling work by Petersen’s team suggests that a study in Liverpool that gave a sensitivity of 40 per cent for LFTs actually indicates that their accuracy is more than 80 per cent.
If rapid LFTs are unreliable, what is the point of taking one?
“Many people seem to view things as either ‘these tests work’, so if I get a negative result then I’m fine, or ‘they don’t work’, so they’re useless and I won’t bother using them, whereas the reality is something in between,” says Hayley Jones at the University of Bristol, UK.
People should use a positive result as a “red light” for stopping a social activity, but they shouldn’t use a negative result as a “green light” to go ahead with an activity that is potentially risky, as they might have some mild covid-19 symptoms, she says. “If your ‘pre-test’ decision is not to do the activity, then I would not generally recommend letting a negative result change your mind,” says Jones.
How about if there is only a faint line on an LFT?
That shows virus protein is present. “Positive lateral flow test results can show an extremely faint band, which some people might incorrectly read as a negative test result,” says Jones. “I think [that] has not been well communicated.”
Why are rapid LFTs not recommended if someone has symptoms?
The UK government recommends that people use LFTs only if they don’t have symptoms, for regular screening or on days when they are likely to mix with other people. If anyone has symptoms, they should use the more accurate PCR tests. But in practice, many people do use LFTs if they have symptoms, especially if they are very mild or are not the “official” covid-19 symptoms.
“You can have the [virus] proteins whether you have symptoms or not,” says Petersen. “The test doesn’t know if you have symptoms.” In fact, most studies of LFTs suggest they are somewhat more accurate at detecting positive cases in people who have symptoms than in those who are symptom free.
So is it OK to use an LFT if I have symptoms?
The crucial thing to remember is that if you have covid-19 symptoms, a negative LFT doesn’t give you the all-clear: you still need to use a PCR test. But LFTs for people with symptoms can be helpful while they are waiting for a PCR result, especially if PCR tests are hard to access, as they are at the moment, says Petersen.
I’ve heard LFTs don’t detect the omicron variant, is that right?
This isn’t true. Although the omicron variant has more than 30 mutations, the antibodies used in LFTs can still bind to the viral proteins. “Initial laboratory validation… has determined similar sensitivity to detect omicron compared to delta,” the UK Health Security Agency said last week. Nevertheless, some people will get false negative results because the test’s sensitivity is relatively low.
In some reported cases of people spreading the virus after a negative LFT, they may have been genuinely negative at the time of testing and turned infectious a few hours later. “It is always possible to test negative and then go to positive,” says Al Edwards at the University of Reading, UK. Because of this, it is better to test just before mixing with other people, rather than relying on a negative test from the day before.
Will we get better rapid tests in future?
Several other kinds of rapid tests are either in development or already available. For instance, a product from a UK firm called Intelligent Fingerprinting involves taking a swab from inside the cheek, which is easier than swabbing the nose or throat. Another UK firm called Aureum Diagnostics is developing a system where people can put a saliva sample on a test strip. The strips are impregnated with the ACE-2 receptor, which the coronavirus normally binds to when entering human cells. But to be useful, any new test would have to be as cheap and easy to mass produce as LFTs, says Edwards. “Where LFTs really succeed is their scalability and [ease of] mass manufacturing.”
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