Fowler et al. 1 proposed RNA RT-LAMP as a rapid and accurate tool to promptly identify highly contagious individuals during the pandemic era. In the current vaccination era, it would be useful to have a reliable tool to provide information on the infectivity/contagiousness of individuals. FDA approved antigen (Ag) test as a fast and convenient alternative to PCR but, as known, this approach can be effective at symptoms onset, 2 when viral antigen is abundant, 3 otherwise false negative results can occur; moreover, positive antigenic results need to be confirmed by molecular test 4. These assays are mostly qualitative and, even when a numerical value is provided, no straightforward correlation with the virological and clinical parameters has ever been demonstrated. We evaluated an Ag test based on chemiluminescence (CLEIA), Lumipulse® G SARS-CoV-2 Ag (Fujirebio INC), in an extensive population with different characteristics. This comparative study included 1000 nasopharyngeal samples (NPS), analyzed during the period fall/winter 2020-2021 at the Virology laboratory of Bambino Gesù Pediatric Hospital. NPS were collected in Universal Transport Medium (UTM, Copan) and immediately analyzed for molecular SARS-CoV-2 detection by AllplexTM SARS-CoV-2 Assay (Seegene), according to which, 850 samples resulted positive (Cycle threshold, Ct< 40) and 150 negative. Referring to an external standard curve (y=− 3.179 x+ 42.28; R2= 0.939), defined on the basis of serial dilutions of a commercial standard (EDX SARS-CoV-2 Standard, Exact Diagnostics LLC), for each Ct, the corresponding RNA viral load was calculated and expressed in log10 copies/ml. Antigen detection was performed by Lumipulse® G SARS-CoV-2 Ag, using the automated Lumipulse G1200 System (Fujirebio). Samples were considered negative when SARS-CoV-2 Ag concentration was< 1 pg/ml, in gray zone when≥ 1.00 and< 10 pg/ml and positive when≥ 10 pg/ml, according to manufacturer’s instruction.

Considering molecular test as the reference standard, Ag showed a specificity of 95.33%(143/150 samples resulted negative, 7/150 resulted positive, 6 of which, included in the gray zone) and a sensitivity of 64%(541/850 samples had a SARS-CoV-2 Ag concentration< 1 pg/ml, 131/850 between 1.00 and< 10 pg/ml, and 178/850≥ 10 pg/ml).