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COVID-19 Antigen Testing Explained

Frequently asked questions about COVID-19 antigen testing.

Written by Dr Adam Abbs. Last updated 15/03/2021.

What are the different types of COVID-19 tests?

There are three categories of testing - PCR, antibody and lateral flow (antigen) testing. Click here for more information on PCR testing and antibody testing.

What is lateral flow testing?

Lateral flow tests, or “rapid tests” look for proteins produced by the virus called antigens. This doesn’t require a laboratory, meaning that they can be done quickly and easily, at a school, workplace or even at home. They look like a pregnancy test.

What is a COVID-19 lateral flow antigen test?

In the case of Covid-19, lateral flow tests are known as rapid antigen tests. A swab is used to sweep the inside of the nose and the back of the throat, this is then inserted into a tube of liquid. The proteins from the swab then mix with the liquid, which is then placed onto the pad of the pregnancy-style test kit. The liquid is absorbed by the pad where it comes into contact with antibodies which change the colour of the pad if they meet the antigens associated with Covid-19, this will show up as a coloured line. 

How is an antigen test different from a PCR test?

An antigen test looks for proteins created by the virus, whereas a PCR test looks for the genetic material of the virus itself. Both antigen and PCR tests are looking for a current infection. An antigen test can be done at home, using a pregnancy-style test, giving results in minutes, whereas a PCR test needs large machinery, which can be found in laboratories.

How is an antigen test different from an antibody test?

An antigen test looks for proteins created by the virus, whereas an antibody test looks for the cells that your body has created to protect you from coronavirus (because you have previously been exposed to it). 

An antigen test looks for current infection, whereas an antibody test looks for previous infection. An antigen test can be done at home, using a pregnancy-style test, giving results in minutes, whereas an antibody test requires a blood sample to be sent to the laboratory.

How do you perform an antigen test?

An antigen test involves rolling a swab inside of each nostril. This should be done 10 times per nostril to ensure a good quality sample has been taken. The swab is then placed into a liquid to allow the antigen to dissolve in the liquid. This liquid is then dripped onto the device's pad, producing a reaction that gives a result. This process must be timed for 30 minutes after incubation using a timer or a stopwatch on a mobile phone for example. The result will be visible on the device precisely 30 minutes after the sample is applied. Unlike a PCR test, there is no need to send the sample to a lab. 

Antigen tests are reported to be reliable and can successfully identify whether an individual has COVID-19 despite the absence of symptoms.


Who can get an antigen test?

Antigen testing is currently being offered to those without Covid-19 symptoms and who cannot work from home. This includes NHS staff, care home staff, teaching staff, students allowed to attend higher education institutions and employees of companies that have signed up to the government testing scheme. The majority of antigen testing takes place at asymptomatic test sites, in a range of community settings such as universities, schools, care homes and workplaces. Testing at these sites is assisted: you will swab yourself under the supervision of a trained operator who then processes the test and reads the result. 

How effective is antigen testing?

Lateral flow tests used by the UK government go through a rigorous evaluation by the country’s leading scientists. This means they are accurate, reliable and successfully identify those with COVID-19 who are not showing symptoms and could spread the virus without knowing. Antigen tests deliver a rapid result in 30 minutes and can find positive cases with high levels of virus present that are easy to transmit to others, helping to intercept and reduce further infections. 

How accurate is antigen testing?

Test accuracy is normally measured in sensitivity and specificity. We have explained these below.

Sensitivity 

Sensitivity is how good a test is at finding infection in a person with a condition. When a person has low levels of the virus in their system, antigen tests are less sensitive than some of the other tests used, such as PCR which is used for those experiencing symptoms. This means that antigen tests may not realise someone has a condition if they have smaller amounts of the virus in their airways. They are more reliable when an individual has a high viral load; which is important as these people are more likely to spread the virus. Ongoing review for rapid antigen testing shows an overall sensitivity of 76.8%. This is quite low, so if you have symptoms, we recommend that you use an alternative test, such as a PCR test

Specificity 

Specificity is how good a test is at giving a negative result in a person without a condition. If a person doesn’t have a condition, a test with a high specificity is more likely to correctly identify that they don’t have it. Ongoing review for rapid antigen testing shows that the test has a specificity of 99.68%.

Oxford University and PHE confirm lateral flow tests show high specificity and are effective at identifying most individuals who are infectious.


Can I take an antigen test if I have symptoms of COVID-19?

You should not use this test if you have Covid-19 symptoms, or have had contact with someone with Covid-19 as you may get a negative result which is not correct. This can be dangerous as you may think you are clear when you are spreading the virus to other people. If you suspect that you have Covid-19 symptoms, you should self-isolate immediately, report this to your manager at your place of work and request a PCR test.

Test results

How do I know if my rapid antigen test is positive for COVID-19?

If two lines appear on the device, one next to ‘C’ (Control) and one next to ‘T’ (Test), even if they are faint, the test is positive. Yourself or your test provider must report this test result to NHS Track and Trace. You must also self-isolate immediately to prevent further transmission of the virus.

How do I know if my rapid antigen test is negative for COVID-19?

If one line appears next to ‘C’ on the device, the test is negative. It should be noted that a negative result does not rule out the COVID-19 infection as there can be false-negative results. 

How do I know if my test result is void?

Any test WITHOUT a line next to ‘C’ should be voided and a new test done with a fresh kit. C= Control - it shows you that the test is working. You should see a ‘C’ on every test result.

This diagram shows an example of what an antigen test result might look like.


How long does it take for a test result to appear?

The time it takes for a test result to appear varies for different tests. You should leave your test for a recommended 15-minute development time to get an accurate result. If the test device is left to develop longer you may receive a false-positive result and you will need to repeat the test. 

How do the antigen test kits need to be stored?

Test kits including the test cartridge and extraction solution should be stored at room temperature, 2 to 30℃. 

How do I dispose of the test kit?

You should carefully dispose of the test device and equipment in your household rubbish.

Can I reuse the test?

No. These tests are single-use only.

Medicspot and antigen testing

Does Medicspot offer a private antigen test?

We are working to provide an antigen test service for our customers in the near future and will keep this page updated as and when these developments are made.

Travel and antigen testing

Are rapid antigen tests accepted for travel?

Antigen testing is accepted for some travel, e.g. travelling to the USA and is most commonly used to test if you are able to board a flight to return back to the UK. However, it is important to check that the test you are using to travel is accepted by both the airline(s) you’re travelling with and the country you’re travelling to, before you purchase the test. You’ll also need to check that the test provider is UKAS accredited and that you’ll get a certified ‘fit to fly’ certificate where appropriate. Medicspot uses only UKAS accredited laboratories for it’s Covid testing services.

Travel abroad should be limited unless deemed essential. It is important to follow strict national guidelines by maintaining the use of face coverings, keeping a safe distance from others and disinfecting hands often.


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