Rapid antigen tests

Maximilian Reindl studied chemistry and biochemistry at the LMU in Munich and has been a member of the editorial team since December 2020. He will familiarize himself with medical, scientific and health policy topics for you in order to make them understandable and comprehensible.

More posts by Maximilian Reindl All content is checked by medical journalists.

Rapid antigen tests detect acute infections with the Sars-CoV-2 pathogen. Your advantage: You deliver quick results. That is why they are particularly suitable for use in hospitals, nursing homes, schools or at work. Infection chains can thus be identified and interrupted at an early stage. Read everything about rapid corona antigen tests - how they work and how reliable they are.

What is a rapid antigen test?

A rapid antigen test is used to detect an existing coronavirus infection. The test detects certain proteins of the Sars-CoV-2 pathogen in the sample material.

Such a test is used for acute diagnosis and is one of the direct detection methods for Sars-CoV-2. The result of a rapid antigen test is a snapshot of your state of health. A past coronavirus infection cannot be detected with rapid antigen tests.

The structure is reminiscent of a pregnancy test: You apply a sample to a small test field (sample window) and after a short time you receive visual feedback. A result is usually shown in the form of two lines.

Unlike the more complex but more reliable PCR test, a rapid antigen test delivers a result after 15 to 30 minutes. No laboratory is required for evaluation.

As part of the national test strategy, they represent a useful addition to more complex PCR tests in order to quickly discover infection clusters and quickly interrupt infection chains.

Rapid antigen tests are also known as point-of-care tests (POC).

Biggest limitation in application: rapid antigen tests can only reliably detect “large amounts” of virus material in a sample.

Therefore, please note: Most patients develop typical Covid-19 symptoms around four to five days after an infection. This means that larger amounts of virus material are usually only present in the secretions of the upper respiratory tract from the onset of symptoms.

If the amount of virus in the sample falls below the detection limit (detection limit), you may receive a false negative result.

A negative rapid antigen test is therefore not a free ticket. Despite a negative rapid antigen test, continue to obey the AHA rules and wear your FFP2 mask. For yours and the protection of others.

When does a rapid antigen test make sense?

Antigen self-tests should help to quickly confirm a suspected Sars-CoV-2 infection or to rule out an infection as far as possible.

They are particularly useful in situations in which a quick result is important in order to assess the current infectiousness. They are therefore suitable for regular screening in schools, kindergartens or at work - they can also help to determine a possible risk of infection in the run-up to a meeting with family or friends.

How is the test used?

Rapid antigen tests are usually performed at designated testing centers, pharmacies, or general practitioners' offices. However, not every pharmacy offers such a test - there is usually insufficient capacity for this.

The sample for a rapid antigen test is usually taken by trained people. Carrying out the (deep) nasal or throat swab correctly is important because the virus concentration is highest in the back of the throat - in the case of an infection. If the smear is carried out incorrectly, the result of a rapid antigen test can be falsified.

Please note: You can find more information on the subject of coronavirus self-tests in our topic special under the following link.

For the smear, the trained staff insert a sterile test stick into your nostril and wipe the back of your throat. The test stick is then immersed in a tube with extraction buffer so that the (possibly) existing viruses are transferred into this solution.

In a further step, one to three drops of this sample solution are usually added to the test field of the test cassette. The test result can be read after 15 to 30 minutes.

A visual feedback now appears in the upper part of the result window. A colored control line must always appear - this is usually marked with the letter C. It shows whether the test sequence was correct. If the control line does not appear, the test is invalid.

In the lower part of the results window there is another marking: the test line (T). If it appears too, the test is considered positive. There is then a high probability that you are infected.

What do I do if the result is positive?

If you have received a positive result from a rapid antigen test, further clarification is necessary. A subsequent PCR test confirms and then backs up the preliminary results.

It is mandatory to report positive rapid antigen tests carried out in a test center, at a doctor's or at a school. This means that the responsible health department receives a notification in this case and usually orders a quarantine.

However, if the additional PCR test is negative, the quarantine that has been ordered is lifted again.

If you have carried out a rapid antigen test for your own use (self-test) and received a positive result, there is currently no official reporting requirement. Act responsibly, contact your trusted doctor by phone or call 116 117.

Reliability of the rapid antigen test

How reliable the result of a rapid antigen test is in practice depends on a few factors that you should consider when evaluating a rapid antigen test result:

• When to take the sample: A rapid antigen test can give false negative results before the onset of symptoms. It could be the case that you are infected, but the test turns out negative ("sensitivity gap").
• Quality of the sample: A professionally performed (deep) nasopharynx swab increases the chances of a correct test result. Incorrect sampling can falsify the result.
• Product quality: Each rapid antigen test differs in terms of quality and the respective product specifications. In general, the following applies: The higher the specified sensitivity, the higher the hit rate for identifying infected people as actually infected. The higher the specificity of a rapid antigen test, the higher the accuracy that healthy people are recognized as actually healthy.

The Federal Institute for Drugs and Medical Devices (BfArM) maintains and continuously updates a list of all rapid antigen tests used in Germany that meet the minimum criteria according to the Coronavirus Test Ordinance.

Who pays for the rapid antigen test?

All citizens are entitled to a free rapid antigen test per week.

This regulation has been in effect since March 8, 2021. These tests are carried out in the test centers of the local health authorities or by commissioned third parties - such as general practitioners and pharmacies.

Further (professional) rapid antigen tests are possible - however, you will have to bear the costs yourself. The additional offers cost an average of 25 to 45 euros.

How does a rapid antigen test work?

A rapid antigen test is an examination method from bioanalytics. Specifically, it is a so-called immunochromatographic test.

The principle of this procedure: Specific antibodies recognize an antigen (to be examined). If this antigen is in the sample, it triggers a downstream (enzymatic) color reaction.

What does a rapid antigen test detect?

In a corona antigen rapid test, the so-called nucleocapsid of the Sars-CoV-2 pathogen is (usually) detected. This is a specific viral protein structure that is only present in the sample when the coronavirus is actively multiplying in your body.

Coronaviruses consist of four so-called structural proteins: spike protein (S), envelope protein (E), membrane glycoprotein (M) and nucleocapsid (N).The spike protein and the nucleocapsid are particularly suitable for antigen detection.

How is a rapid antigen test structured?

A rapid antigen test usually consists of three different antibody variants that were previously produced in the laboratory - i.e. biological protein structures with special properties: They are designed in such a way that they precisely recognize a target structure - the antigen - and thus bind it (at the molecular level) .

They selectively bind the target structure, which means that they can differentiate molecular structures - such as the nucleocapsid, for example - from other biological structures in the sample.

So there are within the test cassette:

• "Freely moving" antibodies: They only recognize and bind to the nucleocapsid. After the sample has been applied, they migrate in the carrier medium from the sample window to the test and control zone. If the virus is in the sample, they form what is known as the antigen-antibody complex.

• Fixed antibody in the "test zone" (T): It specializes in recognizing only the antigen-antibody complex - but not the freely moving antibody. If virus protein is in the sample, only the antigen-antibody complex attaches itself to the fixed antibody in the test zone (T). As a result, a color reaction starts there.

• Fixed antibody in the "control zone" (C): It specializes in only recognizing (excess) freely moving antibodies. But not the antigen-antibody complex. Regardless of whether the sample contains the coronavirus or not, a color reaction sets in. If done correctly, a control line will always appear. If it does not appear, the test is considered invalid.

In summary: If there is sufficient coronavirus in the sample, the presence of the coronavirus is indicated by a colored reaction product in the test zone (T).

A pregnancy test (in the form of a urine test) works in the same way. A pregnancy test therefore also belongs to the group of "rapid antigen tests". However, no virus proteins are detected there, but the presence of the pregnancy hormone chorionic gonadotropin (hCG).

When is a rapid antigen test negative?

However, if the antigen you are looking for is not in the sample or if it has fallen below the detection limit, only the control line is formed in such a case.

The detection limit is the smallest amount of virus particles that can still be reliably detected in a sample to be examined.

Tags:  fitness sports fitness foot care