SARS-CoV-2 IgG Antibody Quantification

Quantitative evaluation of SARS-CoV-2 IgG anti-spike (S) glycoprotein antibodies was performed by the Liaison^® SARS-CoV-2 TrimericS IgG chemiluminescent immuno-assay (CLIA) on the Liaison XL (Diasorin^® S.P.A., Saluggia, Italy), according to the manufacturer’s instructions. The cut-off for positivity was 33.8 binding activity units (BAU)/mL. This assay showed an optimal correlation with the micro-neutralization test (negative and positive agreement of 100% and 96.9%, respectively) and was standardized against the WHO internal standard [42 (link)]. Regarding analytical performance, high sensitivity (98.7%) together with high specificity (99.5%) ensure accurate results. Samples containing levels of IgG anti-S antibodies above the measurement range (>2080 BAU/mL) were further diluted 1:10 using LIAISON^® TrimericS IgG Diluent. In order to exclude previous SARS-CoV-2 asymptomatic infection during the overall period considered, the anti-N response was determined using the Roche Elecsys^® Anti-SARS-CoV-2 electro-chemiluminescence immuno-assay (ECLIA) on the Cobas e 601 module (Roche^®, Mannheim, Germany).