Covid: How accurate are lateral flow tests? What does test sensitivity mean?

Lateral flow tests have emerged as one of the frontline defences in preventing the spread of covid-19, but are often criticised as being less accurate than PCR tests. Here’s why.

By Liv McMahon
Wednesday, 29th December 2021, 10:45 am
How accurate are lateral flow tests? What does test sensitivity mean? (Image credit: guaschfotografic/inkdrop via Canva Pro)
How accurate are lateral flow tests? What does test sensitivity mean? (Image credit: guaschfotografic/inkdrop via Canva Pro)

While efforts to speed up booster vaccinations across the UK have now seen almost 60% of Scots over the age of 12 receive a booster dose, regular lateral flow testing remains a vital way to limit the spread of Omicron as daily coronavirus cases soar.

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Also known as lateral flow devices or rapid antigen tests, lateral flow tests should be used by those not displaying any common covid-19 symptoms and taken at least twice a week, every three to four days.

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The tests offer results within just half an hour, and as such have quickly evolved as a quick and easy (if not slightly uncomfortable) way of identifying whether you have Covid-19.

But questions around the accuracy of lateral flow tests have grown with our increased reliance on them.

Here’s what you need to know.

How accurate are lateral flow tests?

According to the NHS and UK Government, research carried out in March 2021 indicated that lateral flow tests are at least 99.9% accurate – with a minimal chance of seeing a false-positive result from one of these tests.

But reports from the British Medical Journal (BMJ) have said that based on studies from UK-based health institution Cochrane, there is reason to suggest that this might not be the full picture.

A review published by Cochrane on March 24 this year suggested that lateral flow tests could be better at identifying people with symptoms rather than those who are asymptomatic.

The Cochrane study, led by University of Birmingham Professor Jon Deeks, found that for those confirmed to have coronavirus, lateral flow antigen tests correctly identified the virus in 72% of people with symptoms.

When it came to those showing no Covid symptoms, however, the devices only identified 58% of those confirmed to have coronavirus.

This has led to some confusion over the accuracy of lateral flow tests and their ability to produce a higher proportion of false negative results than PCR tests.

What does test sensitivity mean?

Measuring the accuracy of tests such as lateral flow or PCR tests takes place by determining their sensitivity, meaning how likely a test is to identify whether someone actually has an illness or infection such as Covid-19.

The Cochrane review, which pooled together a range of studies on lateral flow test accuracy, is among many which have found that lateral flow tests generally offer less sensitivity than PCR tests – which have been described as the gold standard in coronavirus testing, while not always 100% accurate.

The slightly reduced sensitivity of lateral flow tests, measured at 76.8% for all PCR positive individuals in a joint study by Public Health England (now the UK Health Security Agency) and the University of Oxford, can lead to higher rates of false negative test results.

For instance, tests with a sensitivity of 75% can be expected to provide accurate, positive results for 75% of those with an infection or illness.

But the remaining 25% of people could likely see a false negative result returned when they have actually been infected with the disease or illness tested for.

Sensitivity is measured alongside a test’s ‘specificity’ for which lateral flow tests are thought to perform more accurately.

Specificity is measured by a test’s ability to accurately diagnose uninfected individuals, with a higher specificity reducing the possibility of false positive test results.

Lateral flow test devices present an estimated specificity of 99.68%, according to the Public Health England and University of Oxford study, meaning false positives are less common than false negatives.

So scientists and health officials have urged that a negative lateral flow test result should still not be considered absolute confirmation of not having the virus.

PCR tests therefore remain an important way to confirm a positive lateral flow result and diminish the risk of false negatives if you have any Covid or Omicron-like symptoms, with their greater sensitivity and a specificity of over 95%.

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