Evaluation of ECLIA antigen detection tests as screening methods for COVID-19 in comparison with molecular analysis

Background: Data from literature shows that antigen tests are rapid and helpful tools for diagnosis of COVID-19. Aim: This work aimed to evaluate the performances of the Elecsys SARS-CoV-2 Antigen test, in comparison to RT-qPCR, the gold standard. Methods: A total of 110 swabs were tested; according to rRT-PCR, 76 were positive, and 34 were negative. The swabs were processed by Elecsys SARS CoV 2 Antigen assay (Roche Diagnostics GmbH, Mannheim, Germany), an electrochemiluminescence immunoassay (ECLIA). Results: In a first evaluation, the overall sensitivity and specificity were 85% and 100%, respectively. It was noted that most of the discordant cases had cycle threshold (Ct) values > 28. Therefore, it was assumed a new measure to evaluate sensitivity and specificity, then samples with Ct values < 28 were selected. In this way, it was achieved a Ct < 28 sensitivity of 94%. The level of agreement between the two tests was 89. 1% with κ value of 0.77 for total data and 95.9% with κ value of 0.95 for samples with < 28 Ct. The antigen test performs well in the presence of high viral loads, whereas lower levels are missed. Conclusions: The comparison data obtained in this study support that this method seems a proper approach for rapid screening of patients with high SARS-CoV-2 viral load; however, the rate of sensitivity is highly Ct-dependent.