With the aid of an antigen test, viral components of the SARS-CoV-2 virus can be detected in a sample taken. This sample material is obtained, for example, via a nasal or throat swab. The sample is first brought into contact with a test liquid so that the virus components can be dissolved from the swab and transferred to the liquid. This test liquid is then applied to the respective antigen test and then slowly migrates over the entire test strip. This takes an average of 15 minutes. If viral components are present in the sample, they react with colored molecules at the detection line and a color band is formed. This results in a qualitative detection of SARS-CoV-2 virus components (antigens). A second color reaction at the control line ensures that the test is functional.
The PCR test also detects viral components, but this is the genetic material - the viral RNA. This must first be extracted from the sample in the laboratory and is then amplified for detection. This allows even a small amount of viral RNA to be detected, making this method very sensitive. However, due to the processing in the laboratory, this test method is very time-consuming and cost-intensive.
An antibody test checks whether a person has already experienced a corona infection and whether the immune system has therefore formed antibodies against the virus. This does not detect an acute infection, but only checks whether a person has already been infected with the virus.
The viral load excreted during an infection with SARS-CoV-2 is low, especially at the beginning of the infection and when it subsides. In these phases, the probability of infecting other people is low, but it is also more difficult for the test systems to provide reliable results. The colour reaction of the antigen tests only takes place when a certain amount of virus is present, which is why the most reliable results are obtained in the most infectious phase. Therefore, the test result is only considered a snapshot, as there is a possibility that the viral load was too low when the test was performed and this can change again a few hours later. Therefore, even after a negative test result, one should continue to follow all hygiene measures and carry out antigen tests at regular intervals.
What should I do if the test result is positive?
You must immediately self-isolate and inform the responsible public health authorities. In Germany, you can also contact the on-call health service (116117). They can inform you about the steps you should take and can give you an appointment for a PCR test to confirm the antigen positive result.
What should I do if the test result is negative?
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.
If no control line appears:
This means that the test result is invalid. Please repeat the test with a new test kit according to its instructions leaflet.
What do the sensitivity and specificity of an antigen test mean?
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.
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.