Importance of anti-SARS-CoV-2 assay antigenic composition as revealed by the results of the Belgian external quality assessment (EQA) scheme

Moerman, Alena; Vernelen, Kris; China, Bernard; Capron, Arnaud; Bossche, Dorien Van Den; Mariën, Joachim; Ariën, Kevin K.; Van Acker, Jos; Delforge, Marie-Luce; Reynders, Marijke; Boel, An; Depypere, Melissa; Van Gasse, Natasja; Vijgen, Sara; Brauner, Jonathan; Dujardin, Barbara; Padalko, Elizaveta

BACKGROUND: We report on sample IS/17575 since it generated highly divergent results in the Belgian SARS-CoV-2 serology EQA scheme. MATERIALS AND METHODS: Sample IS/17575 was serum originating from a 30 years old male patient. 124 diagnostic laboratories analysed this sample. RESULTS: A total of 168 results was returned (including 5 doubles). Overall, 38 were positive. All tests against S1 were positive except the Euroimmun IgG ELISA and the Ortho clinical Diagnostics VITROS IgG CLIA. All tests against S1/S2 (Liaison, Diasorin) resulted in a signal above cutoff. Assays against RBD, mostly generate a negative result. An exception are the Wantai SARS-CoV-2 ELISA's. All tests targeting N protein were negative. CONCLUSION: The survey shows, when > 6 months post-infection, assays targeting at least S1, and preferably S1 combined with S2, are the most sensitive. This finding accentuates the necessity of EQA schedules and antigenic composition of serologic SARS-CoV-2 assays.