Comparison of the clinical performance of two SARS-CoV-2 antigen testing with rapid chemiluminescent immunoassays

ABSTRACT

Background. One of the strategies suggested for the containment of SARS-CoV-2 pandemic is testing at risk populations with rapid turn-out of results, contact tracing and isolation of infected individuals, at least until vaccination programs have been completed. Molecular testing of naso-pharyngeal swabs (NPS) is considered gold-standard, but antigenic testing should offer the advantage of being more rapid. Aim. The aim of this study was to evaluate the clinical performance of two chemiluminescent immunoassays on laboratory automated platforms, LIAISON® SARS-CoV-2 Ag Assay (DiaSorin) and Elecsys SARS-CoV-2 Antigen (Roche), to detect SARS-CoV-2 N antigen in NPS. Patients and Methods. A total of 281 subjects were consecutively enrolled (116 M, 165 F) from three different cohorts 14 were COVID-19 in-patients (Group 1), 149 were patients enrolled at the emergency unit (Group 2) while 118 were healthcare employees under SARS-CoV-2 periodic surveillance (Group 3). All subjects underwent NPS with eSwab Copan. Antigen and molecular testing were performed soon after collection. Results. Thirty subjects were SARS-CoV-2 positive at molecular testing. Liaison antigen was positive (>200 TCID50/ml) in 22/30 (Se=73.3%), equivocal (100-200 TCID50/ml) in 4/30 and negative (<100 TCID50/ml) in 4/30 subjects. Specificity was 61.8% since 60/157 negative samples had equivocal results. With Elecsys sensitivity was 75.9% and specificity 99.5%. ROC curves were performed to compare the two assays and to identify the best cut-off. The areas under the curves were not different (η2=0.14;Prob>η2=0.7077). The highest likelihood ratio for Liaison corresponded to 150 TCID50ml cut-off, while for Elecsys to 1 index value. With these thresholds sensitivity of these two assays were 86% and 87% respectively, with 99% specificity. The limitations in sensitivity were due to false negative results for samples with Ct values at molecular analysis higher than 25. No false negative case was recorded among those with Ct lower than 25. Conclusions. In conclusion NPS SARS-CoV-2 antigen testing with chemiluminescent immunoassays allows the rapid detection of positive samples with a sensitivity and specificity that meet the recommendations of the WHO for this type of testing.