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Covid-19 Test Accuracy

People have been asking about the ‘lateral flow’ testing now being used across the UK for students, workers and volunteers. So, our researchers have done this handy guide for you. BuDS has used only reliable scientific and medical sources to write this article and it has been reviewed by our expert team, which includes a clinical epidemiologist.

There are two main types of Covid test.

 

RT-PCR TESTS – MAINLY ACCURATE, BUT SLOW

The diagnostic test used in hospitals and testing laboratories is the RT-PCR or ‘reverse transcriptase polymerase chain reaction’ test. This is a highly sophisticated test which extracts genetic material from a sample and uses a special test machine to make that genetic material rapidly duplicate itself. If the genetic material is part of the Covid-19 virus, the machine detects this and indicates that Covid-19 was found in the sample.

The RT-PCR test is very accurate for positive results. If the samples are taken by trained professionals, it is over 99% accurate. If the samples are taken by a trained member of the public, this falls to a still very respectable 95% accuracy. This means that if you get a positive result, you almost certainly had Covid-19 when the test was taken.

The RT-PCR test is also accurate for negative results. If the samples are taken by trained professionals, is it around 76% accurate. This means that if you get a negative result, it is likely – but not certain – you didn’t have Covid-19 when the test was taken. The higher the quality of the sample, the more accurate this estimate is. However, because of this, if you receive a negative test result but still have symptoms of Covid-19, you should still isolate as if you were infected, along with getting another test to confirm the result.

However, the RT-PCR test is expensive and needs specialist staff and equipment. This is why people can only get a RT-PCR test by sending a sample swab away in the post to be processed at one of the huge testing centres set up by the NHS. This takes a few days to do.

 

LATERAL FLOW TESTS – FAST, BUT MOSTLY INACCURATE

‘Lateral flow’ or ‘lateral flow antigen tests’ are quite different from PCR tests. Millions of test kits rather like a pregnancy test have been produced in factories all over the world. Inside the kit is a chemical solution that detects specific proteins found in Covid-19. The sample is taken by a swab from a person’s mouth and nose and put into the test kit. Within about 30 minutes, the kit shows a coloured line which indicates that the specific Covid-19 proteins have been detected.

The advantage of ‘lateral flow’ test kits is that they are easy to mass produce and quick to give a result. This means lots of people – especially people who are not showing any symptoms – can be quickly tested to see if they test positive. If they do, they can be isolated immediately until a PCR test can prove whether or not they have Covid-19. Lateral flow tests therefore offer a really good way of quickly removing potentially infectious people from society and so reducing the spread of Covid-19. This is why school pupils, university students, teachers, workers and volunteers are all being regularly given lateral flow tests, to quickly remove potentially infectious people who could infect everyone else.

The disadvantage of lateral flow tests is that they are *very* inaccurate. There is as much chance that a lateral flow test is wrong as it is right, but it is hard to gauge the exact inaccuracy. According to the Government, who carried out analysis on tests they had already bought, around 20%-40% of tests were inaccurate. Media reports today (15 April) on Government testing appear to confirm this, although the actual data has not yet been published. However, according to widespread real-world community testing, including at the University of Birmingham in a student population, between 40% and 97% of tests were inaccurate. This is shocking, and shows just how inaccurate the tests can be in real-world situations. If you want to know more about these studies, BuDS will be publishing an article soon on our website discussing it in detail – see links below for more.

So, whatever the lateral flow test shows, you must get that result confirmed by a PCR test before you can be sure that it is right. This applies whether the lateral flow test shows you have got Covid-19 or not. It also applies even if you have had several lateral flow tests – they could all be inaccurate.

 

A NEGATIVE LATERAL FLOW TEST DOESN NOT MEAN YOU ARE FREE FROM COVID-19!

The important thing to remember is that a negative lateral flow test result does NOT prove that you are free from Covid-19. These tests do NOT prove that someone is safe to attend school or work because they do not have Covid-19. BuDS is very concerned that many people, schools, workplaces and organisations are treating lateral flow tests as if they were a way of finding people who are safe to go to school or work. This is entirely false and unscientific. The tests were never designed to do this.

Everyone should still be treated as potentially infectious even if they have had a negative lateral flow test. They should wear facemasks, use social distancing and hand washing to protect other people from catching Covid-19 from them. Other people, especially high risk people like older or shielding people, should not trust a lateral flow test result to prove that they can start getting close to people again. Be safe and stay safe!

 

MORE INFORMATION

BuDS will be publishing a more in-depth article soon looking closer at the statistics and facts around the different types of tests. Look out for it at https://buds.org.uk/articles-about-covid-19/ (this link will be updated when the article is published).

 

FINALLY

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