Rapid Antigen Run-down

What are rapid antigen lateral flow tests, how do they work and are they any good?

Dr Shohaib Ali BSc MBBS, Dr Alasdair Scott MBBS MRCS PhD

November 9th, 2020

Coronavirus testing is constantly developing and “lateral flow tests” or “rapid antigen tests” have been appearing more and more in mainstream media. But what are these tests and how do they work? How good are they and why does there seem to be debate about them? This article will give you the run-down.

What is a rapid antigen or lateral flow test?

Simply put, a rapid antigen test is a quick test to detect small parts of the coronavirus (antigens) which are present in a swab or saliva sample. The actual technology used in most of these tests is called “lateral flow” so lots of people just call them lateral flow tests. This technology has been used for many years and is actually how home pregnancy tests work.

Rapid antigen tests rely on taking a sample from the nose or throat or a saliva sample which would likely contain coronavirus if a person had the infection. A buffer solution is added to the sample and a few drops of this mixture are placed onto one end of the test “cassette”, after 10-15 minutes the result can be read as either positive or negative for coronavirus infection.

A video of how the test works in more detail can be found here.

How accurate are rapid antigen tests for coronavirus?

To determine how accurate rapid antigen tests are for detecting coronavirus, scientists working run a clinical validation study. In these studies, they take a group of people with typical symptoms of coronavirus and test them with both a PCR test and an antigen test. Patients with a positive PCR test are considered to have coronavirus while those with a negative PCR test are considered to be clear of the virus. Investigators then see how well the results of the antigen tests agree with the PCR test.

Different brands of rapid antigen test will all have different accuracy figures. For example, we currently use a swab test by Healgen / Orient Gene. Their clinical validation study found that for every 100 positive PCR tests, we would expect approximately 97 positive antigen tests. This means the sensitivity of the test is 97%. Similarly, for every 100 negative PCR tests, we would expect 99 negative antigen tests. The specificity of the test is 99%. So you can see that the antigen test agreed with the PCR test in the vast majority of cases.

However, it’s important to realise that these studies are carried out on people with symptoms of coronavirus who are likely to have a relatively high amount of the virus. Also, the swabs were taken by healthcare professionals. In reality, these tests are being used for screening people without symptoms and people are often taking their own swabs. People without symptoms are generally likely to have low viral amounts of the virus and they may not do their nose swab adequately. In these cases, it’s likely that the antigen test will not pick up coronavirus as well as the more sensitive PCR test. However, we don’t really have much data on this kind of testing yet.

The bottom line is that a positive result from an antigen test means it’s likely you have coronavirus but a negative test result doesn’t guarantee you don’t have it.

What’s the difference between rapid antigen and PCR tests?

Rapid antigen tests are different from from Polymerase Chain Reaction (PCR) tests. PCR tests work by detecting the genetic material (called “RNA”) of the coronavirus in a sample, rather than antigens, This requires laboratory equipment and generally takes a few hours. Because PCR tests require specialist equipment and trained staff they are relatively expensive.

PCR is very sensitive and will detect tiny amounts of genetic material in a sample. On the one hand this is really good because PCR can tell when someone has the virus even if they have a very low “viral load” (meaning they don’t have much of it), which might be before they have even developed symptoms. On the hand, it may not be so good because PCR tests can remain positive for weeks (or even months) after someone has recovered from coronavirus and is no longer infectious to others because small amounts of genetic material can remain in a person’s nose for a while.

Why is there so much interest in rapid antigen testing?

One of the many reasons that coronavirus has spread so quickly is because around 1 in 3 people who have the virus don’t display any symptoms like fever or cough. These people don’t feel sick so will go about their day-to-day lives as usual, potentially spreading the virus to others who do get sick. Identifying people who have coronavirus, but don’t have any symptoms is a crucial step in stopping the spread of this virus. This is where rapid antigen tests come in. Because they are low-cost, quick and can be done at home, they could be taken regularly by everyone, helping to pick up infections in people without symptoms who would not otherwise have known they even had the virus. It’s not practical or effective to do this kind of screening with PCR testing because it takes too long to give a result and is too expensive.

What does this mean for me?

It’s likely that we’ll see rapid antigen tests being used more and more in the fight against coronavirus. The NHS and businesses are already using them on their staff and some countries are permitting them for travellers as an alternative to PCR testing. We may well start to see them being used at events or other social gatherings.

However, rapid antigen tests are not going to replace PCR which will remain the gold standard test, particularly for individuals with symptoms. They’re also not going to stop us having to take the usual physical precautions against coronavirus such as hand-hygiene, face-masks and social distancing.

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