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.

Waning but persistent humoral response 6 months after the third dose of the mRNA BNT162b2 vaccine in hemodialysis and peritoneal dialysis patients

“…According to the manufacturer’s protocol, patients with an anti-spike antibody titer below 0.8 AU/mL were classified as seronegative. An anti-spike antibody titer below 257 AU/mL—corresponding to the threshold of the WHO International standard unit of 264 binding antibody units [BAU]/mL[ 5 ] which offers 80% protection against symptomatic COVID-19 [ 6 ]—was classified as a low antibody titer. The kinetics of the humoral response was assessed as the ratio of the difference in anti-spike antibody titer between M1 and M6, over the titer at M1, and expressed in percentage.…”

In vitro Characterization of Anti-SARS-CoV-2 Intravenous Immunoglobulins (IVIg) Produced From Plasma of Donors Immunized With the BNT162b2 Vaccine and Its Comparison With a Similar Formulation Produced From Plasma of COVID-19 Convalescent Donors

“…Moreover, the concentration of these antibodies was higher in VP-IVIg than in CP-IVIg ( Figure 4 ). The concentration of anti-RBD antibodies in VP-IVIg and CP-IVIg corresponds to 7902 ± 69 and 3109 ± 199 binding antibody units (BAU)/mL, respectively ( 30 ). Results of ACE2-RBD neutralizing antibodies in CLIA were further confirmed using a PRNT ( Figure 5 ), where the observed IC 50 concentrations were lower for the VP-IVIg (mean: 0.05 g/L; 95% confidence intervals (CI):0.03–0.09) than for the CP-IVIg (mean: 0.09 g/L; 95% CI: 0.06–0.13).…”

Highly Sensitive and Specific SARS-CoV-2 Serological Assay Using a Magnetic Modulation Biosensing System

“…Overall, the analytical performance of the 45-min MMB-based assay is better than the ELISA test, and is achieved with a ~5.4-fold improvement in the turnaround time (45 vs. 245 min). Using the WHO international standard for the anti-SARS-CoV-2 IgG antibody to compare the MMB-based assay and the ELISA test to other commercially available assays, such as Mindry, Roche, DiaSorin, Thermo-Fischer, and Euroimmun [ 30 ], the MMB-based assay has a similar dynamic range and a relatively low cutoff value ( Supplementary Materials, Table S4 ).…”