Home DiseasesCovid-19 Rapid antigen tests: application, reliability, function

Rapid antigen tests: application, reliability, function

by Josephine Andrews
Published: Last Updated on 328 views

Rapid antigen tests detect acute infections with the pathogen Sars-CoV-2. Your advantage: You deliver quick results. They are therefore particularly suitable for use in hospitals, nursing homes, schools or at work. Chains of infection can thus be detected and interrupted at an early stage. Read everything about rapid corona antigen tests here – how they work and how reliable they are.

What is an antigen rapid test?

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

Such a test is used for acute diagnostics and is one of the direct detection methods for Sars-CoV-2. The result of an antigen rapid test therefore represents a snapshot of your state of health. A previous coronavirus infection cannot be detected with a rapid corona antigen test.

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 displayed in the form of two lines.

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

Rapid antigen tests are also referred to as point-of-care tests (POC, POC antigen rapid test).

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

Therefore, please note: Most patients develop typical Covid 19 symptoms about four to five days after infection. This means that large amounts of viral 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 limit of detection (limit of detection), you may get a false negative result.

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

When is a rapid antigen test useful?

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

They are particularly useful in situations where a quick result is important to assess the current infectivity. 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 applied?

Rapid antigen tests are usually performed at designated test centers, pharmacies, or family doctor’s offices. However, not every pharmacy offers such a test – there is usually not enough capacity for this.

The sample for an antigen rapid test is usually taken by trained people. It is important to carry out the (deep) nose or throat swab correctly, since the virus concentration is highest in the back of the throat – if an infection is present. If the swab is carried out incorrectly, the result of an antigen rapid test can be falsified.

Please note: Further information on the topic of coronavirus self-tests can be found in our special topic under the following link .

For the swab, the trained staff inserts a sterile test stick into your nostril and wipes the back of your throat. The test stick is then dipped into a tube with extraction buffer so that the (possibly) present viruses pass into this solution.

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

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 indicates whether the test procedure was correct. If the control line does not appear, the test is invalid.

At the bottom of the result window is another marker: the test line (T). If she also appears, the test is considered positive. You are then more likely to be infected.

What do I do if the result is positive?

If you have received a positive result from an antigen rapid test, further clarification is necessary. A subsequent PCR test then confirms and secures the provisional finding.

There is an obligation to report positive rapid antigen tests carried out in a test center, by a doctor or at a school. This means that the responsible health authority receives a message in this case and usually orders a quarantine.

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

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

Reliability of the antigen rapid test

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

  • Time of sample collection: Before the onset of symptoms, a rapid antigen test can provide false-negative results. So it could happen that you are infected, but the test is negative (“sensitivity gap”).
  • Quality of the sample: A professionally performed (deep) nose and throat swab increases the chances of a correct test result. Incorrect sampling can falsify the result.
  • Product quality: Each antigen rapid 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 recognizing infected people as actually infected. The higher the specificity of an antigen rapid test, the greater the certainty that healthy people will be recognized as actually healthy.

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

How long are rapid antigen tests valid?

Rapid antigen tests are used for acute diagnostics. A negative test result is therefore only valid for 24 hours after the swab.

According to current knowledge, there are no fundamental differences in the detection quality for most of the antigen rapid tests available on the market.

This means that the omicron variant is also recognized by the rapid antigen tests (recommended by the Federal Office for Drugs and Medical Devices). The Paul-Ehrlich-Institut also continuously checks whether there will be any deviations in this assessment in the future.

Who bears the costs of the rapid antigen test?

All citizens are still entitled to a free Covid-19 test once a week (probably until June 30, 2022) using a rapid antigen test – according to the currently applicable coronavirus test ordinance, even without symptoms. 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.

Other (professional) rapid antigen tests are possible – but you have to bear the costs yourself. The additional offers cost an average of 25 to 45 euros.

How does a rapid antigen test work?

An antigen rapid test is an examination method from bioanalysis. 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 subsequent (enzymatic) color reaction.

What is detected in a rapid antigen test?

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

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

How is an antigen rapid test structured?

A rapid antigen test usually consists of three different antibody variants that have been 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 bind the target structure selectively, which means they can distinguish molecular structures – such as the nucleocapsid – from other biological structures in the sample.

The test cassette therefore contains:

  • “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 the so-called antigen-antibody complex.
  • Fixed antibody in the “test zone” (T): It specializes in only recognizing 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 begins there.
  • Fixed antibody in the “control zone” (C): It specializes in only recognizing (excess) freely moving antibodies. But not the antigen-antibody complex. A color reaction begins regardless of whether the sample contains the coronavirus or not. When performed correctly, a control line always appears. If it does not appear, the test is considered invalid.

In summary: If the sample contains a sufficient amount of coronavirus, 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 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 is 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.

You may also like

Leave a Comment