Coronavirus antibody tests – what are the differences and which ones are worth taking?

Anti-SARS-CoV-2 antibodies are produced by the immune system of a person infected with the SARS-CoV-2 virus, and in order to determine the patient’s condition, they need to be tested by performing the appropriate serological tests. There are several types of such tests, which we decided to introduce in the following post.

What are antibodies and what is their role in the human body?

The main role of antibodies, produced by the immune system, is to protect the human body from threats, which can be bacteria or viruses, among others. Their presence in a patient’s blood is one of the most crucial pieces of information about an infection.

Due to their mode of action, antibodies are divided into five classes – IgG, IgM, IgA, IgE and IgD. In the case of SARS-CoV-2 virus infection, antibodies from the first three classes are the most important. The IgG class is a group of the most persistent antibodies, which can remain in the human body long after infection. In contrast, antibodies from the IgM class are formed the fastest of all types and are ultimately replaced just by IgG antibodies. Among other things, the IgA class is associated with respiratory immunity, which is why it is so important for coronavirus infections.

By performing tests to determine the presence of anti-SARS-CoV-2 antibodies in the body, it is possible to determine whether the patient has already passed the disease asymptomatically (IgG class) or is still infected (IgM or IgA class). A positive serological test result should be confirmed by a genetic test, performed by Real-Time PCR.

Covid antibody tests

Tests detecting anti-SARS-CoV-2 antibodies can be divided into several types due to the method of performing the test, the way the results are presented or the testing of specific antibody classes. Importantly, depending on the manufacturer of the test, their diagnostic specificity, that is, the ability of the test to indicate uninfected individuals, is between 98.5 and 100%. In contrast, sensitivity, or the ability of the test to detect infected individuals after the tenth day from the onset of symptoms, is between 70 and 100%.

Read: How does the antigen test for COVID-19 work? How does the RT-PCR differ from a respiratory swab?

What are the differences between coronavirus antibody tests?

Division by the way the result is presented:

• Qualitative tests without class division are characterized by high accuracy. Specialists from the British Institute of Public Health laboratory verified this method by verifying tests prepared by Roche. According to their results, they verified correctly 100% of the samples that came from people with COVID-19 disease. Other tests they verified also showed the presence of antibodies unrelated to coronavirus, but only similar to COVID-19 disease. Roche tests are screening tests that detect the presence of antibodies in the human body using an automated qualitative method without class division. It is a method that has very good specificity and high analyzer performance. Any positive test result is a prelude to further diagnosis. A negative one, on the other hand, indicates a negligible risk of active infection and a history of infection.
• Quantitative: Detection of coronavirus antibodies is also possible using a quantitative method on an automated analyzer. Such a test uses equipment from DiaSorin, among others, which detects IgG neutralizing antibodies directed against the antigen of the SARS-CoV-2 virus. This solution makes it possible to obtain a quantitative result that shows the exact concentration of antibodies in the blood. The test material is the patient’s venous blood. This type of testing is recommended when we want to check what proportion of a large group of people, such as employees of one company, has already been infected and thus likely to have acquired immunity to the SARS-CoV-2 virus.

Division taking into account antibody classes:

• No breakdown by class: Detection of anti-SARS-CoV-2 antibodies without class division can be performed by an automated qualitative method using a Roche analyzer or cassette method, among others. The material collected for the test is the patient’s blood from a vein or finger. The patient receives a result given as plus or minus, but without a detailed indication of which antibodies are present in the blood. This is a test that is worth performing as a screening test of a larger number of patients, as well as for periodic monitoring of their epidemiological status.
• By class (IgM/IgA or IgG): Coronavirus antibody tests by class can be performed on automated analyzers. Then the different antibody classes provide information about ongoing infection (in the case of IgM or IgA antibodies) and past history of the disease (IgG antibodies). This test is more accurate than the variant without class division, as it can be used to verify, among other things, whether the patient has already experienced the disease in an asymptomatic or sparse manner. Testing of young antibodies (IgM or IgA) is best done at least 7-10 days after contact with a potentially infected person, while IgG antibodies after 14 days.

Which coronavirus antibody test is worth doing?

The choice depends on a number of factors, but we especially recommend paying attention to qualitative tests that give information about the patient’s health status. In the case of a positive result, we recommend performing a semi-quantitative antibody test in the IgM class and a quantitative one in the IgG class. Such a set of tests is a reliable information on SARS-CoV-2 virus infection, in addition, it has a very good price-quality ratio. For those who get a positive result in the IgM class, we recommend performing a genetic test using the RT-PCR method, which will give an answer as to whether the patient currently has COVID-19.

We do not mention the RT-PCR method here, as it is a genetic test for which material is taken from the nose or throat. However, people who have symptoms such as shortness of breath, cough or fever should use it to verify their condition. RT-PCR is also at the end of the testing pathway in the case of positive anti-SARS-CoV-2 antibody tests, in which case it may turn out that a person who does not have symptoms is actually sick, which is what the genetic test can confirm. Its performance should take place 4-7 days after contact with a potentially infected person.