Background: There are significant number of tests used to determine the level of antibodies to SARS-CoV-2 which
differ both in the methods underlying testing and in the antigenic targets used and classes of measured immunoglobulins. Comparison of the results obtained using various tests reveals their significant discrepancy when
converted to the WHO recommended standard unit for measuring the level of specific immunoglobulins BAU/
mL. The aim of this study is a comparison of anty-SARS-CoV-2 IgG levels, measured using test systems based on
different methodological platforms - EuroImmun assay and Abbott assay.
Method: Abbott uses the immunochemiluminescence method CLIA, EuroImmun uses the enzyme immunoassay
method ELISA. The dependences of the measurement error on the level of antibodies for the two test systems
were approximated by power functions using the least squares method. The nonlinear relation of antibody levels
values measured by Abbott assay and Euroimmun assay was approximated by an asymptotic function.
Results: The study involved 112 people. Our results confirm the fallacy of using a single conversion coefficient in
BAU/mL for anti-SARS-CoV-2 IgG levels measured by Abbott and EuroImmun. To describe the interdependence
of anti-SARS-CoV-2 IgG Abbott and EuroImmun levels, we offer the function y = 18/π arctan(0.0009x) and a
calculator that allows to easily recalculate the results obtained using these tests.
Conclusion: The non-linear nature of the interdependence of the measured anti-SARS-CoV-2 antibodies levels on
the levels magnitude is one of the main reasons for the discrepancy between the tests results when converted to
BAU/mL using a single conversion coefficient.