The WHO International Standard for COVID-19 serological tests: towards harmonization of anti-spike assays

Abstract

Background and aims: SARS-CoV-2 antibody assays are relevant in managing the COVID-19 pandemic, providing valuable data on the immunization status of the population. However, current serology tests are highly variable, due to their different characteristics and to the lack of reference materials. The aim of the World Health Organization (WHO) first International Standard (IS) for anti-SARS-CoV-2 immunoglobulin is to harmonize humoral immune response assessment after natural infection or vaccination, and recommend reporting the results for binding activity in Binding Antibody Units (BAU).

Materials and methods: This study analyzed six commercial quantitative anti-SARS-CoV-2 S-protein assays in a head-to-head comparison, using the manufacturers' conversion factors for the WHO IS to obtain BAU/mL values.

Results: Our data showed good alignment up to 1000 BAU/mL, then began to disperse, exhibiting some discrepancies. Moreover, correlations among methods varied with Cohen's Kappa ranging from 0.580 to 1.00, with the lowest agreement values for kits using different target antigens or different antibody isotypes, making it clear that the laboratory report should include this information. Values expressed as BAU/ml showed a reduced between-assays variability compared to AU/ml (median coefficients of variation 0.38 and 0.68, respectively; p < 0.001).

Conclusion: On the basis of these data at present anti-SARS CoV-2 serological assays' results are not interchangeable, and, more importantly, individual immune monitoring should be performed with the same method.

Keywords: COVID-19; COVID-19 Serological Testing; Harmonization; Serological tests; WHO international standard; anti-SARS-CoV-2 antibodies.

Fig. 1

Scatter-plot for each serological assay in BAU/mL. Data were sorted from the lowest to the highest value. Dotted line represents the value of 1000 BAU/mL. On the right an enlargement of the figure shows a good alignment up to about 1000 BAU/mL. Samples who have reached a plateau, represented by their maximum linearity range, have been excluded.

Fig. 2

Two-test pair correlation data for the serological assays. The straight line represents the linear fit; the coefficient of correlation (R), the number of samples (n) and the level of significance (p) are also indicated.