How accurate are lateral flow tests?

Covid cases continue to rise in the UK

Laura Hampson
Thursday 28 July 2022 12:03 BST
How to take a lateral flow test

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Latest figures show that around 3.8m people in the UK, approximately one in 17, has Covid.

Data from the Office for National Statistics (ONS) released earlier this week found that this was a 7 per cent rise on cases the week before.

However, most people living in England, Wales and Scotland can no longer access free Covid testing, and virus restrictions have been largely lifted across the country under the government’s plans to live with coronavirus.

Free lateral flow tests are only available to eligible groups - such as NHS staff, adult social care staff, and those at serious risk of illness from Covid. Other members of the public must purchase lateral flow test kits if they wish to test at home.

But how accurate are these at-home tests in determining whether or not you have Covid?

According to a meta-analysis of studies by medical database the Cochrane Library, lateral flow tests detect an average of 72 per cent of symptomatic cases and 58 per cent of asymptomatic ones – which could be the reason why you’re testing positive one minute and negative the next.

Michael Head, senior research fellow in global health at the University of Southampton says: “Lateral flow tests are very good at producing a positive result when you are infectious (and so able to transmit the virus). This can include the day or two before symptoms develop. Thus, they are a very good first line of inquiry, to be followed up with confirmation via the more sensitive PCR test.”

A study published in the Clinical Epidemiology journal in October 2021 from researchers at University College London, Liverpool University, Harvard University and the University of Bath found that lateral flow tests are more than 80 per cent effective at detecting any level of Covid-19 infection and are more than 90 per cent effective at detecting Covid when people are at their most infectious.

A study from UKHSA published in December 2021 found that lateral flow tests are as effective at detecting the Omicron strain of the virus as they are the Delta, which means whatever strain you may have the test should be able to pick up that it’s Covid.

“A Cochrane review of 64 studies found that lateral flow tests correctly identify 72 per cent of infected people who have symptoms, and 78 per cent within the first week of becoming ill,” GP Dr Gary Bartlett says.

“What I often tell my patients is that lateral flow tests are really useful for asymptomatic screening (testing when you have no symptoms). If you have Covid symptoms, lateral flows are less reliable as they can be associated with false negatives often leading the person to believe that they don’t have Covid when in fact they do.”

Head adds: “False positive results will happen in a fraction of a per cent of occasions, but false negatives (i.e. a negative result when in fact you do have Covid-19) are more common. Therefore, if you have Covid-like symptoms but test negative on a lateral flow, it is a good idea to book in for a PCR test.”

A faint line on the ‘T’ indicates a positive Covid test
A faint line on the ‘T’ indicates a positive Covid test (iStock)

What is the correct way to take a lateral flow test?

Hussain Abdeh, clinical director and superintendent pharmacist at Medicine Direct says, if you can, you should avoid eating and drinking anything for at least half an hour before you take a lateral flow test.

“Similarly, do not smoke or vape for as long,” he continues. “Make sure the surface you are taking the test on is disinfected and make sure the kit is not damaged when you take it out of the box. Wash or sanitise your hands before handling any of the apparatus. Blow your nose then wash your hands again before starting.”

Each lateral flow test kit will come with instructions on how to use it. If you need to carry out a throat swab, Abdeh says to open your mouth wide and rub the swab over the back of your throat – but don’t let it touch your tongue, gums or teeth.

Following the throat swab, insert the same swab into your nose “until you feel a little resistance” and swab it around for about 10 seconds. Some tests will require you to take a nose swab only.

Once the swab is complete, insert the cotton end of the swab into a tube of liquid and press the swab down to the bottom of the tube. “Squeeze the bottom of the tube and move the swab around so that the sample can be properly transferred into the liquid,” Abdeh advises.

“Remove the swab from the tube, close the tube’s cap and shake the liquid around a little to mix the sample with the liquid. Squeeze the stated number of drops of liquid onto the test strip and check the strip after the stated amount of time has passed.”

The government website says it’s best to leave the test for 15 to 30 minutes before checking the result.

Does a faint line on the “T” mean you’ve tested positive?

If the result is negative for Covid-19, there will be a singular red line next to the “C” mark on the lateral flow test. A positive result means there is a red line next to both the “C” mark and the “T” mark – but what if the second red line is faint?

“When reading lateral flow test results, two lines through both the ‘C’ and ‘T’, even faint lines, indicate the test is positive,” a Public Health England spokesperson says. “However, a ‘T’ line alone indicates the test has failed and will need to be retaken.”

Is it possible to have a negative lateral flow test and a positive PCR test?

Public Health England says lateral flow tests and polymerase chain reaction (PCR) tests have “different roles to play” in controlling the virus, so we can’t directly compare them.

The spokesperson adds that lateral flow tests are useful for finding out if the person is infected now and able to transmit the virus to others. “The level of sensitivity is high enough to detect most of these cases,” they said. “Lateral flow tests are less likely to return a positive result outside the infectious window.”

A PCR, on the other hand, is used to confirm or deny a suspected case of Covid-19.

“The higher sensitivity of PCR tests means it can identify genetic material from Covid-19 even after the active infection has passed,” Public Health England adds.

If you have Covid symptoms but test negative using a lateral flow test, it’s worth ordering a PCR to double check.

How effective are lateral flow tests in combatting the spread of Covid-19?

According to the NHS, one in three people with Covid-19 do not show symptoms but can still infect others, which is why taking a lateral flow test regularly – even when you’re vaccinated – is so important in helping to stop the spread of the virus.

The infection period for Covid-19 can start up to two days before symptoms begin to show and last for 10 days afterwards.

Those who are cautious could take a lateral flow test twice a week, but if you’ve been in contact with someone who has caught Covid-19 you may wish to do a daily lateral flow test for the seven days following.

“Lateral flow tests are highly effective at preventing the spread of Covid,” Abdeh says. “First of all, you can get them ordered to your home, so if you have come into contact with someone who has tested positive, or if you are exhibiting possible symptoms, you can avoid spreading it to others by leaving the house.

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