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Antigen tests, like PCR tests, are used for the direct detection of pathogens and thus the detection of an acute infection. Proteins are detected that are components of the pathogen that are very specific for this pathogen. If the target antigen is present in sufficient concentration in the sample, it binds to specific antibodies and generates a signal on the test strip. The result is purely qualitative. Currently, when "rapid tests" are mentioned, antigen tests are usually meant. Antigen tests are mostly used to test asymptomatic persons, for example, those returning from travel to risk areas, before visiting nursing homes, etc. A positive antigen test should be confirmed by PCR test.
Antibody tests do not detect the virus itself, but the antibodies as a reaction of the immune system to the pathogen. The immune system starts working when the virus enters the body and forms antibodies after a few days. These are part of the defence system and are easily detectable in the blood. Because of the time delay and the presence of the antibodies beyond the infection, an antibody test is not suitable for detecting an acute infection.
The name is due to the characteristic wreath-shaped appearance ( Latin "corona" : wreath, crown ) of coronaviruses, which is due to the spike protein, the barbed surface protein . Coronaviruses can cause harmless colds, but are also the causative agents of Severe Acute Respiratory Syndrome (SARS) and MERS, which stands for Middle East Respiratory Syndrome. During the first SARS epidemic (SARS-CoV-1) in 2002 and 2003, in which the virus was transmitted to humans through a zoonosis - presumably from bats - 774 people died worldwide, according to the WHO. The new coronavirus SARS-CoV-2, first detected in China in late 2019, can lead to the disease COVID-19.
The viral infectious disease COVID-19 manifests itself as a respiratory disease with symptoms such as cough, shortness of breath, fever or pneumonia. With increasing age and concomitant diseases, the risk of severe courses increases. On 30 January 2020, the WHO initially declared a "public health emergency of international concern" due to the worldwide spread of the virus. Since mid-March, COVID-19 has been classified as a pandemic.
In medicine, immunochromatography is often used as an analytical method, e.g. in rapid tests such as pregnancy tests or antigen tests. Immunochromatography is based on a solid liquid-absorbing material in which a specific antibody is bound. It is directed precisely against the substance (antigen) that is of interest in the context of the respective test (e.g. SARS-CoV-2). The antigen sought is bound by the antibody at a specific site and made visible by a dye (often colloidal gold). This leads to a coloured stripe that indicates whether the test is positive or negative.
The incubation period is the time between infection and the onset of the disease. According to the RKI, studies have shown that 95 per cent of those infected had developed symptoms after ten to 14 days.
Colloidal gold is the term used to describe brines (colloids) or gels made of tiny gold particles with a diameter of 2 to 100 nm. Colloidal gold is characterised by its deep red colour, which is due to the small size of the gold particles. Colloidal gold is often used as a dye in antigen tests (red test strips).
A pandemic is when a disease affects the whole world, i.e. it is not localised. The WHO declared the spread of Covid-19 a pandemic on 11.3.2020.
PCR tests are still considered the gold standard for detecting acute infection with SARS-CoV-2. They are usually performed in the laboratory or in testing centres. A PCR is used for direct pathogen detection using viral RNA. A PCR also allows conclusions to be drawn about the viral load, which can be an indication of the infectiousness (Ct value). Due to the higher effort and the associated costs, PCR tests are usually not used for the untargeted testing of asymptomatic persons.
RNA (abbreviation for ribonucleic acid) - also called RNA (ribonucleic acid) - is a nucleic acid, i.e. a chain of so-called nucleotides. While DNA (deoxyribonucleic acid) stores the genetic material of humans in cells and does not release it, RNA ensures genetic information transfer.
With a rapid test, no analysis of the sample in the laboratory is necessary; instead, the result can be read off after a short time on a test cassette similar to a pregnancy test. Strictly speaking, the term does not indicate whether it is an antibody or antigen test. However, when "rapid tests" are mentioned, they are usually antigen tests for the detection of an acute infection.
The sensitivity of SARS-CoV-2 tests describes the probability of a positive test result that is correctly identified as positive. Sensitivity is the probability that you will get a positive test result if you are actually positive.
Example of use: The sensitivity of an antigen test is approximately 96.5%. This means:
- 96.5% of the test results are positive and you are really positive.
- 3.5% of the time you get a positive test result, but you are actually negative (false positive).
This is why positive test results should always be checked with a more accurate PCR test in the laboratory.
The specificity of tests describes the probability that a negative test result is really negative. The specificity therefore says: with what probability you get a negative test result and are actually negative.
Example of use: The specificity of a rapid antigen test is approximately 99.7%. This means:
- 99.7% of the time you get a negative test result and you are really negative.
- Then in 0.3% you get a negative test result but are actually positive (false negative result). As you can see, the specificity of a rapid antigen test is very good, which is why retesting is not usually recommended here.
A large proportion of all pathogens that can cause disease in humans are transmitted from animals to humans; these can be viruses, bacteria, parasites, worms or prions. Infection can occur through direct contact with animals, contaminated food or through vectors such as ticks and mosquitoes. Examples of zoonoses are BSE, plague, malaria and SARS-CoV-2.
