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22.02.2021

Corona Antigen Rapid Tests 

Background

Detecting SARS-CoV-2 infections early is essential to break chains of infection. That is why it is now possible to perform rapid tests at home. But how do the antigen tests actually work? And what do you have to consider when using one? Keep reading to find out.

What are the different testing procedures?

PCR test

With PCR tests, the sample is examined in a lab to detect any potential genetic virus material. This method is very sensitive and able to detect a low viral load. This means that the SARS-CoV-2 infection can be detected early in the course of the disease. However, PCR tests have some disadvantages –they are expensive and results take a long time.  

PCR rapid test

These tests use the same method as PCR tests, but in a more simplified way. The advantage of a PCR rapid test is that it does not have to be carried out in a lab, so it is much more flexible and convenient. However, it is not as sensitive as the standard PCR test and therefore somewhat less accurate.

Rapid antibody test

This test can be used to check whether a person has already been infected and has already formed antibodies against the pathogen. These tests cannot be used to determine whether someone is currently contagious. 

Rapid antigen test

Rapid antigen tests detect viral proteins. Samples don’t need to be analysed in a lab, which is why these tests are cheaper and much faster than a PCR test –with a rapid antigen test, the result is available in about 15-20 minutes. However, it can only detect an infection if the viral load is high. They are easy to use and can be performed at home.  

How does a rapid antigen test work?

For antigen tests and PCR tests, the first step is to obtain a sample, e.g. via a nasal swab. The sample is then inserted in a test liquid, which is then applied to the test card. The result is available in about 15-20 minutes. Like in a pregnancy test, the results are displayed on the card with different lines. If the test is positive, this means that Coronavirus proteins have been found in the sample. 

When is an antigen test useful?

Thanks to their quick results, antigen tests are a great addition to PCR tests. PCR tests are harder to use and their results take much longer. That is why, if you need a quick result to visit, for example, a nursing home or hospital, antigen tests are much more convenient. Using rapid tests on a regular basis can help detect highly contagious people and prevent further infections. Regular antigen testing can also help keep schools and key sectors open. According to the Robert Koch Institute (RKI), the decline in infections is stagnating (as of February 2021), probably due to the spread of new variants. In order to interrupt this trend, early detection of highly contagious individuals is essential.
It is important to bear in mind that a negative antigen test is no guarantee that one is not infected, because they are not as sensitive as PCR tests. In some cases, the viral load may be too low to be detected with an antigen test, which also means that the person is less contagious.

Sensitivity, specificity… what does it all mean?

The terms sensitivity and specificity are often mentioned in the context coronavirus tests. But what do they actually mean?

Sensitivity

Test sensitivity is the test's ability to correctly identify infected people through a positive test result. For example, if the sensitivity of a test is 98%, this means that 98 out of 100 infected people were detected by the test. However, 2 of those 100 people would get a negative result despite being infected. Such results are called "false negative".

Specificity

Specificity indicates what percentage of uninfected people are detected as such through a negative test. If a test has a 97% specificity, this means that 97 out of 100 people have been correctly identified as uninfected. 3 of those 100 have received a positive test result even though they are not infected. Such results are called "false positives". Most tests have a specificity of 99-100%, which means that positive results in healthy patients are very rare.
To effectively stop chains of infection, it is important to ensure that tests have a high sensitivity and a high specificity.

In addition to specificity and sensitivity, these are some of the other factors to bear in mind when choosing a test: 

Progression of a coronavirus infection and when to test  

The viral load is low at the beginning of an infection and once the infection has subsided. Using a rapid antigen test while the viral load is low can lead to false negative results. However, rapid tests are particularly reliable while the viral load is high, which is when an infected person is most likely to infect others.

Correct sample collection

Sample collection also has a great influence over test results. For example, if the nasal swab is not inserted deep enough, this can affect the result. This can lead to a false negative result if, for example, not enough viral material is extracted. Fortunately, more and more tests are being developed that require a less invasive sample collection, such as spit or more superficial nasal swabs.

What are the different types of sample collection?

Nasopharyngeal swab

With the nasopharyngeal extraction method, the swab is inserted almost horizontally in the nose along the septum, until it reaches the pharyngeal wall. The swab is then rotated a few times and extracted.

Oropharyngeal swab

First, the tongue is pressed down with a spatula. The swab is then inserted in the mouth and rubbed against the pharyngeal wall with a twisting motion.

Both nasopharyngeal and oropharyngeal sample collections are quite uncomfortable and difficult to perform on your own. That is why tests which are designed to be performed at home use non-invasive methods to collect samples. Non-invasive sample collection methods include:

Nasal swab

The swab is inserted into the nose until it reaches the turbinate. Then the swab is twisted and removed. The procedure is repeated with the other nostril.

Saliva tests

Saliva is first loosened from the pharynx –by gargling for 30 seconds or clearing the throat— and then spat into the sample collection container. There are also some methods which collect the saliva directly from the mouth.

Can the rapid antigen tests also detect the new coronavirus variants?

Over the past few months, two new coronavirus variants have been detected. These variants have mutations make them more contagious than the original variant, according to current data. Most of the mutations in these new variants affect the spike protein, the part of the virus that is responsible infecting healthy cells. The rapid antigen tests detect and identify the nucleocapsid protein, not the spike protein. This nucleocapsid protein contains the virus’ RNA. Mutations can also occur in the nucleocapsid protein, but they are a lot less common than spike protein mutations. This means that the new variants from the United Kingdom (B.1.1.7) and South Africa (B.1.351) can still be detected by rapid antigen tests.

What should I do if the result of my rapid antigen test is positive?

If your antigen rapid test result is positive, you should immediately self-isolate and inform the relevant health authorities. They will inform you about the next steps and will possibly instruct you to get a PCR test, to confirm the rapid test result.

If my rapid test result is negative can I be sure that I am not infected?

A rapid antigen test is just a "snapshot". Even if the test is negative, you could be infected but your viral load is still too low to be detected. If this is the case, you could still become contagious shortly after taking the test. That is why you should carry out rapid antigen tests regularly and should continue to follow hygiene measures, such as social distancing, wearing a mask and using hand sanitizer.

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