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.

Tuesday, 13th July 2021, 1:19 pm
Coronavirus: How accurate are lateral flow tests? What does test sensitivity mean? (Picture: Andrew Milligan/PA Media)

As the UK and Scotland prepare for restrictions being eased next week, lateral flow tests are still mandated as one of the first points of call in helping to stop the spread of coronavirus.

Also known as lateral flow devices or rapid covid-19 tests, lateral flow tests should be used by those not displaying any common covid-19 symptoms and taken at least twice a week, every 3-4 days.

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.

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But questions around how accurate 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 recent studies from fellow UK-based health institution Cochrane, there is reason to suggest that this might not be the full picture.

This comes following a review published by Cochrane on March 24 this year, which 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 covid-19, lateral flow antigen tests correctly identified the virus in 72% of people with symptoms.

When it came to those showing no symptoms, the devices only identified covid-19 in 58% of those with confirmed coronavirus.

What does test sensitivity mean?

Measuring the accuracy of tests such as lateral flow or PCR tests takes place by determining their sensitivity, which means finding out how likely a test is to identify whether someone is really infected according to its true positivity rate.

The Cochrane review, which pooled together a range of studies on lateral flow test accuracy, is among several 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.

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The slightly reduced sensitivity of lateral flow tests means that it can be prone to resulting in higher levels of false positives.

Lateral flow devices are viewed as generally offering a lower sensitivity rate and as such, are prone to producing higher levels of false positives.

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

But scientists and health officials are urged that a negative lateral flow test result should still not be considered absolute confirmation of not having the virus, advising that receiving this result should not see people immediately relax their behaviour and commitment to covid-19 safety precautions and rules.

This is because their accuracy can still differ depending on where and how these tests are carried out, with the most reliable lateral flow tests results seen as more reliable when administered by health professionals.

PCR tests therefore remain an important way to confirm a positive lateral flow result and diminish the risk of false positives, with their greater sensitivity and a specificity (true negative rate) of over 95%.

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