Calibration of SARS-CoV-2 antibody assays across blood establishments in Europe

ABSTRACT

Background: SARS-CoV-2 antibody tests are variable using different antigens, reagent dilutions and units of reporting. Without calibration interlaboratory interpretation of assay results is not possible, SARS-CoV-2 antibody tests are variable using different antigens, reagent dilutions and units of reporting. Without calibration interlaboratory interpretation of assay results is not possible, slowing down progress in treatments with convalescent plasma. slowing down progress in treatments with convalescent plasma. Aims: The aim of this study, which is part of the SUPPORT-E consortium (Supporting high-quality evaluation of COVID-19 convalescent plasma throughout Europe), was to calibrate anti-SARS-CoV-2 antibody assays used by European laboratories to determine antibody titers in (convalescent) plasma. Methods: To achieve this we distributed a set of 23 reference samples to 26 participating blood establishments across Europe. This set served as a quality control round for SARS-CoV-2 antibody testing and consisted of SARS-CoV-2 pre-outbreak negative plasma samples and plasma samples from unvaccinated convalescent donors with low, medium, high and very high IgG titers against the receptor binding domain (RBD) of the SARS-CoV-2 Spike (S) protein. In addition, a serial dilution of one batch of pooled plasma (n = 3) was distributed that could serve as internal standard. This methodology allowed calibration to the WHO standard in IU/ml, so conversion factors could be calculated. Results: Twenty laboratories participated, including the qualitative Euroimmune (n = 7), Roche (n = 4) and Abbott (n = 4) SARS-CoV-2 IgG assay as most frequently used tests. Four laboratories, using commercial assays from WANTAI or DiaSorin, were not able to discriminate pre-outbreak samples from SARS-CoV-2 positive sera. In addition, the majority of the laboratories were not able to discriminate between plasma samples with high and very high titers, showing that the dynamic range of these commercial assays is limited. Using these data we calculated the conversion factor to IU/ml for the qualitative Euroimmune as 238, Roche as 0.98 and for the Abbott test as 0.9, which only applies when pre-vaccinated samples are used in the test. Summary/Conclusions: This initiative by the SUPPORT-E consortium aids in calibration of antibody testing across laboratories, allowing to compare SARS-CoV-2 antibody titers in (convalescent) plasma. For example, our conversion factor can now be used to calibrate Euroimmune units that were measured in the convalescent plasma samples within the Recovery (United Kingdom)1 Capsid (Germany)2 and CovEarly (Netherlands)3 clinical trial. .