Covid holidays: What are the differences between Lateral Flow and PCR coronavirus tests? And how do they work?

Travel firms have hit out at the government's plans for people returning from foreign holidays to take an "expensive and unnecessary" PCR coronavirus test.

Under the plans, post-arrival tests must be the polymerase chain reaction (PCR) type which are thought to be more accurate, but are more expensive.

The PCR tests differ from Lateral Flow Tests (LFT) which are widely being used in schools around the country.

So what are the differences between them?

PCR tests require lab analysis meaning they take longer to process. Credit: PA
  • What is the difference between how the tests work?

Polymerase chain reaction (PCR) tests are often seen as the "gold standard" test for identifying clinical cases of infection because it requires laboratory analysis.

The PCR is a diagnostic test that detects the virus’s RNA (genetic material) in a sample taken from a nose and throat swab.

Substances, enzymes known as 'reverse transcriptase' and 'DNA polymerase', are added to the swab to make copies of any viral RNA that is present.

Copies of the RNA are picked up by 'primers' which show the genetic material has been found.

Although they are also swab tests, Lateral Flow Tests are antigen tests and therefore quite different.

Rather than detecting viral genetic material, LFTs detect proteins specific to coronavirus in the sample to detect if people have the virus.

  • Which one is faster?

As LFTs do not require laboratory analysis, they return results faster - usually within 30 minutes - and work similarly to pregnancy tests.

Samples for PCRs have to be sent to a lab so often take between 24 hours to three days to return a result.

  • Why is there a difference in cost?

PCR tests are more expensive due to the analysis that is needed to return a result.

But ABTA, the Travel Association and the Airport Operators Association have also found that UK PCR tests cost double the amount of tests available in other European countries.

It says a pre-departure PCR test in the UK costs on average £128 per person compared to just under £62 in eight key Europe destinations.

It means that people travelling from the UK to Europe would pay an average of £306 for leaving the UK and returning from their destination country under the new traffic light system.

LFTs are cheaper at about £6 to £7 per test and, like pregnancy tests, return a positive or negative result in minutes and can easily be carried out at home.

In England, the government is offering free LFTs to encourage people to get tested twice a week as part of the roadmap out of lockdown.

Health minister Edward Argar said the cost would be covered by the two-year £37 billion NHS Test and Trace budget.

The cheaper - and quicker - Lateral Flow test has been used for mass testing in schools. Credit: PA
  • Which one is more accurate?

Some scientists have expressed concern that rapid LFTs are not as effective as PCR tests especially when people are doing them at home instead of a health professional carrying them out.

Public Health England (PHE) found that the sensitivity of the Innova Lateral Flow test was 79.2% when trained laboratory scientists collected the swab, 73% when it was healthcare professionals but only 57.5% when used by track and trace centre staff.

However, PHE said that as regular mass testing increases with people regularly testing themselves several times a week, this should improve.

The World Health Organization (WHO) has also pointed out that the accuracy of LFTs can be affected according to the time from the onset of infection and the concentration of the virus in the specimen.

It said that LFTs are more likely to detect positive cases when viral loads are the highest and patients are most infectious, typically one to three days before the onset of symptoms and during the first five to seven days after the onset of symptoms.

It could mean that LFTs are less effective at detecting the virus in non-symptomatic people.

"Asymptomatic people have a viral load peak that looks to be, on average, lower than the viral load peak of people with symptoms, and it stays at that peak for less long," Mike Gill, former regional director of public health for the South East of England, said.

However, PCR tests could detect the virus more accurately in people who don't show systems

Alexander Edwards, associate professor in biomedical technology at the Reading School of Pharmacy, said: "We already knew that lateral flow tests do appear more accurate with patients who have more virus present.

"It follows that they may be better suited to spotting ‘spreaders’ than identifying everyone infected."

However, according to the British Medical Journal (BMJ), PCR tests are also not perfect.

It says they can detect viral shedding long after the infectious period, with people continuing to test positive for a mean of 17 days.

"This means that people who are not infectious could be unnecessarily quarantined," it said.

A staff member at a Liverpool school preparing for mass LFT testing of pupils last month. Credit: PA
  • What about false positive or false negative results?

There have been reports that schoolchildren have tested positive on LFTs only for a negative to be returned by a PCR test.

Neither test is infallible but it is thought that the 'sensitivity' of the PCR tests makes false positives and negatives less likely, according to scientists.

Last year, the Scientific Advisory Group for Emergencies (SAGE) said that both the false negative and false positive rate in PCRs were unknown but said accuracy could be effected in real life conditions as "there may be inefficient sampling, lab contamination, sample degradation or other sources of error". 

Last month, the government said Lateral Flow tests produce fewer than one false positive in every 1,000 tests with analysis showing a specificity of 99.9%.

The Department for Education says that when a pupil’s LFT has been taken under supervision, at tests sites or at school, "the chance of it being incorrect is minimal so there is no need for a further test to confirm the result."

But there is a “slightly higher chance” of a positive result being wrong if the test was taken at home, so those should be confirmed by a PCR.

According to the BMJ, Jon Deeks of the University of Birmingham’s Institute of Applied Health Research estimated that false positives in Lateral Flow tests when there is a low prevalence of infection could mean that up to half of positive tests could be false.

"[It] would indicate that half of the children, teachers, families, and their bubbles being asked to isolate this week are doing so unnecessarily," he said.

Free Lateral Flow Tests are available at walk-in centres. Credit: PA
  • Where can I get a test?

LFTs are available for free to anyone whether they have symptoms or not.

These tests are available at local hubs, pharmacies, some workplaces or home kits can be ordered online.

PCR tests are being offered free by the government for people displaying symptoms, for those who have been in contact with someone testing positive, for essential workers or for people who have been asked to get tested by a GP or hospital.

To self-refer for a PCR test, register here for a home test or an appointment at a walk-in centre.