Comparison of a rapid fluorescence immunochromatographic test with an enzyme-linked immunosorbent assay for measurement of SARS-CoV-2 spike protein antibody neutralizing activity.

Background: SARS-CoV-2 Spike protein Receptor Binding Domain neutralizing antibodies (NAbs-RBD) inhibit the viral binding to angiotensin-converting enzyme 2 (ACE2) receptors. We compared an ELISA and a fluorescence immunochromatography (FIC) method in NAbs-RBD detection after COVID-19 immunization.

Method: Serum samples from healthcare workers (HCWs) vaccinated with BNT162b2 were collected one and four months after the second dose. NAbs-RBD (%) detection was performed using ELISA cPass™ (FDA approved) and FIC n-AbCOVID-19® assays.

Results: Samples from 200 HCWs [median age (IQR): 45(35-53)] were tested with both assays. There was a good qualitative agreement between the two methods [AUC: 0.92(95%C.I.: 0.89-0.94, P-value:0.007)]. NAbs-RBD (%), one and four months after immunization, were significantly lower with FIC compared to ELISA for all age groups (P-value<0.0001). The quantitative comparison between FIC and ELISA detected slight agreement one month after the second dose [(Lin's Concordance Correlation Coefficient (CCC): 0.21(95%CI: 0.15-0.27)] which improved four months after the second dose [CCC: 0.6(95%CI: 0.54-0.66)].

Conclusion: FIC had good qualitative agreement with ELISA in the detection of positive NAbs-RBD (%) and could be an alternative for rapid NAbs-RBD (%) testing.