Validation of a commercially available SARS-CoV-2 serological Immunoassay

Aims: To validate the diagnostic accuracy of a Euroimmun SARS-CoV-2 IgG and IgA immunoassay for COVID-19 disease. Methods: In this unmatched (1:1) case-control validation study, we used sera of 181 laboratory-confirmed SARS-CoV-2 cases and 176 negative controls collected before the emergence of SARS-CoV-2. Diagnostic accuracy of the immunoassay was assessed against a whole spike protein-based recombinant immunofluorescence assay (rIFA) by receiver operating characteristic (ROC) analyses. Discrepant cases between ELISA and rIFA were further tested by pseudo-neutralization assay. Results: COVID-19 patients were more likely to be male and older than controls, and 50.3% of them were hospitalized. ROC curve analyses indicated that IgG and IgA had a high diagnostic accuracy with AUCs of 0.992 (95% Confidence Interval [95%CI]: 0.986-0.996) and 0.977 (95%CI: 0.963-0.990), respectively. IgG assays outperformed IgA assays (p=0.008). Considering optimized cut-offs taking the 15% inter-assay imprecision assessed into account, an IgG ratio cut-off > 1.5 displayed a 100% specificity (95%CI: 98-100) and a 100% positive predictive value (95%CI: 97-100). A 0.5 cut-off displayed a 97% sensitivity (95%CI: 93-99) and a 97% negative predictive value (95%CI: 93-99). Adopting these thresholds, rather than those of the manufacturer, improved assay performance, leaving 12% of IgG ratios ranging between 0.5-1.5 as indeterminate. Conclusions: The Euroimmun assay displays a nearly optimal diagnostic accuracy using IgG against SARS-CoV-2 in a samples of patients, without any obvious gains from considering IgA serology. The optimized cut-offs are fit for rule-in and rule-out purposes, allowing determination of whether individuals have been exposed to SARS-CoV-2 or not in our study population. They should however not be considered as a surrogate of protection at this stage.