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Why do SARS-CoV-2 antibody test results differ by laboratory? “Values ​​are not intercomparable” – We are doing Romania well


Tests for antibodies have aroused the interest of those who want to assess their immune status following SARS-CoV-2 virus infection or vaccination. However, with the performance of laboratory tests, questions such as: what does the test result mean, why do the results differ from one laboratory to another, what value of the antibody titer gives me protection?

Laboratory physicians explain that the results of antibody tests may vary depending on several factors, such as the accuracy of the test itself, the sensitivity and specificity of the test, or the method by which the presence of antibodies is determined. There are also differences between kits that detect anti-protein N (nucleocapsid) antibodies and those that detect anti-protein S (spike) antibodies, proteins that are known to be highly immunogenic, meaning they cause significant antibody production.

What does the test result for antibodies indicate?

If you test positive for the antibody, whether it is IgM or IgG, it means that your body came in contact with the SARS-CoV-2 virus before harvest, either after infection or after vaccination.

In contrast, a negative result indicates either that the body has not come in contact with the SARS-CoV-2 virus, or that the time required for the development of antibodies is insufficient.

Differences between kits that detect anti-N antibodies and those that detect anti-S antibodies

Commercial tests reveal anti-N, anti-S, IgM, IgG or total binding antibodies, depending on the manufacturer and the method used (ELISA, chemiluminescence, fluorescence, etc.).

Commercial kits that detect anti-N antibodies have a variation in structure not specified by the manufacturer, but not being an important molecule in the host organism’s immune response strategy, this does not greatly influence the intercomparability of commercial kits.

In contrast, in the case of kits that show anti-S binding antibodies, there are differences, sometimes major, in the structure of antigenic determinants that are contained in reagents, so also in the type of antibodies detected, which can lead to differences in specificity between kits. and, respectively, between the results obtained.

Qualitative tests and quantitative tests

Some tests for antibodies are qualitative, which means that their result confirms or disproves the presence of antibodies, without detecting the concentration. The result of the qualitative tests is expressed in the form of Index, Ratio or S / CO.

On the other hand, quantitative tests determine the concentration of antibodies and are expressed by arbitrary units, such as AU or U, specific to each manufacturer, or BAU / ml, ie antibody binding units.

Specialists point out that these values ​​are not intercomparable from one manufacturer to another, so the results may differ depending on the laboratory.

In addition, physicians explain that there is still no agreement on the concentration (or titer) of neutralizing antibodies that provide protection against further SARS-CoV-2 infection, which is why serological tests are interpreted with caution, thus: the higher the concentration of binding antibodies than the range set by the manufacturer, the more it is believed that there is an increased neutralization antibody titer, especially in the case of anti-S antibodies.

What does the sensitivity and specificity of a test mean

Sensitivity refers to the ability of the test to identify individuals with antibodies against SARS-CoV-2. Probability of having a positive test if you have specific antibodies. An extremely sensitive test detects most people who have antibodies and few people with antibodies will not be identified by this test (false negative reaction).

Specificity represents the ability of the test to correctly detect patients who do not have antibodies against SARS-CoV-2. Chance to have a negative test if you don’t have specific antibodies. A very specific test will identify most people who do not have antibodies and few people without antibodies identified as positive by the test (false positive reaction).

MedLife provides patients with both individual tests for the detection of anti-S or anti-N antibodies, such as IgM and IgG, and post-immunization packages to assess immune status following vaccination or SARS-CoV-2 infection.

We have been with you for 26 years and we do not stop. Because today, more than ever, medicine means fighting through science and knowledge.

To meet our patients with a wide range of services and tests adapted to current needs, we have aligned ourselves with international standards and opened our own Real Time PCR laboratories to detect the SARS-CoV-2 virus. We have invested in the necessary infrastructure to assess whether the patient has gone through the disease and to test the post-infection or post-vaccination immune response.

We are constantly adapting to the present and preparing for the future. We are doing Romania well.

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