Antibody 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.

Antibody tests for Sars-CoV-2 detect certain antibodies in your blood that indirectly indicate a previous infection with the coronavirus. A coronavirus vaccination usually also leads to a positive test result. The reason for this: In both cases, your immune system forms antibodies against characteristic structures of the Sars-CoV-2 pathogen. Find out here what antibody tests are, how they are carried out and when they are useful.

What are antibody tests for?

Antibody tests allow conclusions to be drawn about an infection with the coronavirus. Doctors can also use them to retrospectively prove the disease progression of Covid 19 disease with few symptoms. In principle, it is also possible to use them to check the effectiveness of the vaccination - however, the informative value is usually limited in this context.

Unlike PCR tests and rapid antigen tests, antibody tests are not suitable for clarifying an acute infection. The immune system only develops detectable antibodies against the virus in the later course of the disease. Doctors do not use the tests to check your current state of health, but to find out whether you were previously infected with Sars-CoV-2.

What is an antibody test?

An antibody test is a method of detecting certain antibodies in your blood. Antibodies themselves are specialized protein molecules that the human body produces to protect against pathogens.

In the case of the coronavirus, doctors look for protein molecules in your blood that are directed against characteristic virus structures (nucleocapsid, spike protein).

Is a positive antibody test valid as proof of immunity?

According to the current legal situation, a sole antibody test is not recognized as an official proof of immunity. It is therefore not planned to store them in the CovPass app. It is uncertain whether this regulation will be adjusted in the near future.

Officially, a person is only considered to have recovered if a positive PCR test was at least 28 days ago.

Am I immune if I had Covid-19?

Immunological data suggest a protective effect of about six to eight months after surviving Sars-CoV-2 infection. Those who have recovered should be vaccinated at least six months after the illness. One vaccination dose is then sufficient.

If the disease has not been confirmed by a PCR test and only an antibody test is available, the Robert Koch Institute (RKI) recommends a complete series of vaccinations.

Does an antibody test make sense after a vaccination?

Whether an antibody test makes sense after a vaccination depends on your health situation. The Standing Vaccination Commission (STIKO) does not currently recommend a comprehensive review of vaccination success with antibody tests.

However, there are certain patient groups for whom a test could be useful. This is especially true if there are certain pre-existing conditions that are associated with a weakened immune system.

Patients with chronic inflammatory diseases - for example Crohn's disease, rheumatoid arthritis or multiple sclerosis - often receive immunosuppressive drugs. Such treatment may weaken the vaccination response. Patients suffering from an acquired immune deficiency could also show a weaker immune response to the coronavirus vaccines.

An antibody test could then be used to check whether an immune reaction has taken place (to a sufficient extent) after the vaccination.

It is best to discuss this with your doctor. In a specific case, he can discuss the further procedure with you and help you interpret the test results.

How does an antibody test work?

Antibody tests are one of the so-called serological tests, for which a blood sample is required from you. Various manufacturers now offer a variety of different antibody tests. There are different procedures:

Rapid antibody test

Some test centers use so-called rapid antibody tests. The test is carried out and evaluated directly in the test station. You will receive your result within 20 to 30 minutes. To do this, two to three drops of blood are taken - usually with a prick in the fingertip.

Outwardly, antibody rapid tests are similar to the structure of an antigen rapid test (lateral flow assay format). They consist of a test cassette with a sample window and a test field that shows the corresponding markings for the test and control zones.

At the moment, only trained personnel carry out such rapid tests. They are not available for home use. The quality of the tests available on the market also varies greatly - the meaningfulness 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 a return kit

You can also do some of the antibody tests offered on the Internet yourself. However, the evaluation takes place in a second step in the laboratory.

With the enclosed lancet you can take a few drops of blood from your fingertip at home and put it on the enclosed dry blood card. You then send this in by post with the return envelope. The laboratory then evaluates your sample and sends you the result.

Antibody test at the doctor

Probably the most reliable way of taking samples is by a doctor or trained medical staff. Your doctor will then send the blood sample to a specialized laboratory. You will usually receive the results a few days later.

Antibody tests that are carried out in an accredited laboratory are considered to be very reliable and accurate, as special detection techniques (ELISA, ECLIA) are used here.

When is an antibody test positive?

It takes a certain time for the body to produce antibodies after a coronavirus infection: Certain antibodies are only present in the blood of the person concerned about seven to fourteen days after an infection or vaccination.

However, meaningful results are usually only obtained three weeks after the suspected infection.

What are antibodies

Antibodies are protein molecules made by your body in response to an infection or vaccination. They appear in the blood or in the area between individual cells.

You can find detailed information on how it works and the individual components of the human immune system here.

What are the tasks of antibodies in our body?

Antibodies recognize certain structures of a damaging exogenous organism, virus or even toxin - called an antigen in technical terms.

Antibodies fulfill the following biological functions:

Neutralization: Antibodies can specifically recognize foreign antigens and attach themselves to them. If an antibody adheres to the surface of an antigen, its damaging function is usually slowed down or even completely prevented.

Antibodies, for example, neutralize the coronavirus by "blocking" the spike protein and preventing the virus from entering the human cell. So it can no longer multiply.

Opsonization: This is the second mode of action of antibodies. After antibodies have recognized and neutralized a pathogen, they simultaneously mark it for phagocytes in the human body. In this way, pathogens can be rendered harmless by the body or removed from the body more quickly.

What types of antibodies does our body produce?

Experts divide antibodies into different classes based on their properties - and the time they appear. A distinction is made between an early and 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:

IgM antibodies: They represent the body's rapid immune response. This early class of antibodies is formed after seven to ten days - but they also disappear again after a certain time. Although they offer a certain protection against a pathogen, the body has to "improve" them further. Specialized cells of the immune system - the B cells - replace them with “more targeted” IgG antibodies after around two weeks.

IgA antibodies: This is also an early class of antibodies that, like IgM antibodies, is 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 only formed after about two to six weeks. These are "matured" antibodies. They recognize, bind and neutralize the pathogen in a more targeted manner than early antibody classes. Long-term immune protection can only be assumed when IgG antibodies are detectable.

By the way: The recommended time interval between two vaccination 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 make the transition from early to mature antibodies ("Affinity Maturation").

A second vaccination (booster vaccination) only makes sense if the immune system produces IgG antibodies after the first vaccination dose. These questions about the optimal vaccination interval are, among other things, part of the approval documents and the approval process for a vaccine at the European Medicines Agency (EMA).

How is an antibody test carried out in the laboratory?

Antibody tests carried out in the laboratory are considered to be particularly sensitive and reliable. They not only enable qualitative statements - for example, whether a certain antibody against the coronavirus is contained in the blood sample, but also make it possible to determine its amount (antibody titer determination).

The most common methods are based on the so-called ELISA principle (enzyme-linked immunosorbent assay).

A further development of this principle is the so-called ECLIA process - an acronym from English that stands for "electrochemiluminescence immunoassay". ECLIA is considered to be a very reliable diagnostic and automated detection method.

A patient sample is combined with artificially produced coronavirus antigens. If the blood of a test person now contains antibodies against the coronavirus, all of these components interact with one another in a well-defined way.

They accumulate and form a large common structure - the so-called "double antigen sandwich immune complex". This has a special property: It has a label that emits light as soon as an electrical voltage is applied (ruthenium marker). This amount of light can This also means that the more light is measured, the greater the amount of antibodies against the coronavirus in the sample.

In this way, a qualitative as well as quantitative antibody titre determination is reliably possible.

Tags:  hair dental care menopause