What is the purpose of antibody tests?
Antibody tests allow conclusions to be drawn about a previous infection with the coronavirus. In some cases, they can also be used by physicians to detect low-symptom Covid 19 disease courses in retrospect. In principle, it is also possible to use them to check the effectiveness of the vaccination – but the significance in this context is usually limited.
Unlike PCR tests and rapid antigen tests, antibody tests are not suitable for clarifying an acute infection. The immune system does not produce detectable antibodies against the virus until later in the course of the disease. So doctors don’t use the tests to check your current health status, but to find out if you had been infected with Sars-CoV-2 earlier.
What is an antibody test?
So in the case of coronavirus, doctors look for protein molecules in your blood that are directed against characteristic viral structures (nucleocapsid, spike protein).
Is a positive antibody test considered proof of immunity?
According to the current legal situation, an antibody test alone is not recognized as official proof of immunity. Therefore, there are no plans to store it in the CovPass app. It is uncertain whether this regulation will be adapted in the near future.
A person is not officially considered recovered until a positive PCR test is at least 28 days old.
Am I immune if I have had Covid-19?
Immunologic data suggest a protective effect of approximately six to eight months after surviving Sars-CoV-2 infection. Those who have recovered should be vaccinated no earlier than six months after illness. One dose of the vaccine is then sufficient.
Is an antibody test useful after a vaccination?
Whether an antibody test is useful after a vaccination depends on your health situation. However, the Standing Commission on Vaccination (STIKO) does not currently recommend a nationwide check of the vaccination success with antibody tests.
However, there are certain patient groups for whom a test might be useful. This is particularly true if certain pre-existing conditions are present that are associated with a weakened immune system.
For example, patients with chronic inflammatory diseases – such as Crohn’s disease, rheumatoid arthritis or multiple sclerosis – often receive immunosuppressive drugs. Such treatment could, under certain circumstances, weaken the vaccine response. Patients suffering from acquired immunodeficiency might also show a weaker immune response to the coronavirus vaccines.
It is best to discuss this with your attending physician. He can discuss the further procedure with you in the specific case and help you interpret the test results.
How does an antibody test work?
Antibody tests belong to the so-called serological tests, for which a blood sample from you is necessary. Various manufacturers now offer a wide range of different antibody tests. There are different procedures:
Antibody rapid test
Some test centers use so-called antibody rapid tests. The test is performed and evaluated directly at the test station. You will receive your result within 20 to 30 minutes. Two to three drops of blood are taken for this – usually by pricking the fingertip.
Currently, only trained personnel perform such rapid tests. They are not available for use at home. The quality of the tests available on the market also varies greatly – the significance of the results is therefore limited.
Sensitivity means the reliability with which a test finds the antibody to be detected.
Specificity means the certainty with which the test determines that the antibody in question is not present in the sample.
Antibody self-tests with send-in kit
Some antibody tests offered on the Internet can also be performed by yourself. However, the evaluation takes place here in a second step in the laboratory.
With an enclosed lancet you take a few drops of blood from your fingertip at home and put it on an enclosed dry blood card. You then send this in by post with the return envelope. The laboratory will then evaluate your sample and send you the result.
Probably the most reliable way to collect a sample is by a physician or trained medical professional. Your doctor then sends the blood sample to a specialized laboratory. You will then usually receive the results a few days later.
Antibody tests performed in an accredited laboratory are considered very reliable and accurate because special detection techniques (ELISA, ECLIA) are used.
When is an antibody test positive?
It takes time for the body to produce antibodies after a coronavirus infection. Only about seven to fourteen days after infection or vaccination are certain antibodies present in the blood of the affected person.
Such detections often achieve reliable results only three weeks after a suspected infection.
The different tests can also be based on different evaluation procedures. For example, the results could be given as “ratio values” (so-called ratio values). This means that the result is given as a ratio of a sample to be tested, compared to a reference sample. Accordingly, a value less than 0.8 describes a negative test, a value greater than 1.1 a positive result.
Alternatively, the result can also be given as an absolute value (antibody titer). Laboratories then often give the result in the unit BAU/ml (“binding antibody units” per milliliter). The exact threshold values at which a test (in this unit) is considered positive are currently under discussion. A threshold value between about 20 – 40 BAU/ml is assumed. Any values greater than this transition range indicate a high (or higher) level of protective antibodies.
What does an antibody test cost?
What are antibodies?
Antibodies are protein molecules produced by your body in response to infection or vaccination. They are found in the blood or the area between individual cells.
Detailed information on how the human immune system works and its individual components can be found here.
What are the functions of antibodies in our body?
Antibodies recognize certain structures of a harmful foreign organism, virus or even toxin – called antigen in technical language.
In doing so, antibodies fulfill the following biological functions:
Neutralization: antibodies can recognize and attach to foreign antigens in a targeted manner. If an antibody adheres to the surface of an antigen, its damaging function is usually slowed down or even completely prevented.
Opsonization: This is the second mode of action of antibodies. After antibodies have recognized and neutralized a pathogen, they simultaneously mark it for scavenger cells of the human body. This enables the body to render pathogens harmless more quickly, or to remove them from the body.
What types of antibodies does our body produce?
Experts divide antibodies into different classes based on their properties – and the time of their appearance. An early immune response is distinguished from a late immune response. The latter is characterized by so-called IgG antibodies, the former by so-called IgM and IgA antibodies.
Experts refer to the transition from the early to the late immune response as so-called seroconversion. An important indicator of (existing) immunity is the late immune response: the IgG antibodies.
The following antibody classes are involved in the (humoral) immune response:
IgA antibodies: Also an early class of antibodies that, similar to IgM antibodies, are part of the first defense against a pathogen. They also disappear over time and are replaced by more targeted IgG antibodies.
IgG antibodies: they are considered immunity markers. This late class is formed only after approximately two to six weeks. They are “matured” antibodies. They recognize, bind and neutralize the pathogen in a more targeted manner than early antibody classes. Only when IgG antibodies are detectable can long-term immune protection be assumed.
Incidentally, the recommended interval between two vaccine doses is based on this knowledge of the time course with which the different antibody classes are formed. The body needs a certain period of time to complete the transition from early to mature antibodies (“affinity maturation”).
How is an antibody test performed in the laboratory?
Antibody detections performed in the laboratory are considered to be particularly sensitive and reliable. They not only allow qualitative statements to be made – for example, whether a certain antibody against the coronavirus is contained in the blood sample – but also enable their quantity to be determined (antibody titer determination).
The most common methods are those based on the so-called ELISA principle (enzyme-linked immunosorbent assay).
A further development of this principle is the so-called ECLIA method – an acronym that stands for “electrochemiluminescence immunoassay”. ECLIA is considered a very reliable diagnostic and automated detection method.
A patient sample is mixed with artificially produced coronavirus antigens. If the blood of a test person now contains antibodies against the coronavirus, all these components interact with each other in a well-defined manner.
Thus, a qualitative as well as quantitative antibody titer determination is reliably possible.
