Clinical assessment of FREND COVID-19 Ag test in an unselected population referred for routine SARS-CoV-2 testing

Background: this observational retrospective study was aimed at evaluating the clinical performance of the novel microfluidic fluorescence immunoassay FREND COVID-19 Ag test in a population of unselected individuals undergoing routine SARS-CoV-2 (severe acute respiratory coronavirus 2) testing. Methods: the study population consisted of a series of outpatients referred to the Service of Laboratory Medicine of Pederzoli Hospital (Peschiera del Garda, Verona, Italy) between April 12 and 30, 2021, for SARS-CoV-2 testing for being either symptomatic or having had close contact with one or more COVID-19 cases. A routine nasopharyngeal sample was collected at hospital admission and analyzed with both molecular (Altona Diagnostics RealStar® SARS-CoV-2 RT-PCR Kit) and antigen (FREND COVID-19 Ag) tests. Results: the area under the curve (AUC) of FREND COVID-19 Ag in all nasopharyngeal samples compared to molecular testing was 0.69 (95%CI, 0.64-0.75). At the ≥1.0 TCID50/mL manufacturer’s cut-off, accuracy, sensitivity, specificity, negative (NPV) and positive (PPV) predictive values were 61.3%, 0.27, 1.00, 0.55 and 1.00, respectively. The AUC of FREND COVID-19 Ag in samples with cycle threshold (Ct) values of both SARS-CoV-2 S and E genes <29.5 was 1.00. At ≥1.0 TCID50/mL (median tissue culture infective dose per mL) manufacturer’s cut-off, accuracy, sensitivity, specificity, NPV and PPV values were 99.2%, 1.00, 0.99, 1.00 and 0.95, respectively. Conclusions: FREND COVID-19 Ag could not replace routine molecular testing for achieving a definitive diagnosis of SARS-CoV-2 infection, but can be used as a surrogate test for identifying patients with higher nasopharyngeal viral load and thus greater infectious potential.