Feasibility and Effectiveness Assessment of SARS-CoV-2 Antigenic Tests in Mass Screening of a Pediatric Population and Correlation with the Kinetics of Viral Loads

Abstract

The gold standard for diagnosis of SARS-CoV-2 infection has been nucleic acid amplification tests (NAAT). However, rapid antigen detection kits (Ag-RDTs), may offer advantages over NAAT in mass screening, generating results in minutes, both as laboratory-based test or point-of-care (POC) use for clinicians, at a lower cost. We assessed two different POC Ag-RDTs in mass screening versus NAAT for SARS-CoV-2 in a cohort of pediatric patients admitted to the Pediatric Emergency Unit of IRCCS-Polyclinic of Sant'Orsola, Bologna (from November 2020 to April 2021). All patients were screened with nasopharyngeal swabs for the detection of SARS-CoV-2-RNA and for antigen tests. Results were obtained from 1146 patients. The COVID-19 Ag FIA kit showed a baseline sensitivity of 53.8% (CI 35.4-71.4%), baseline specificity 99.7% (CI 98.4-100%) and overall accuracy of 80% (95% CI 0.68-0.91); the AFIAS COVID-19 Ag kit, baseline sensitivity of 86.4% (CI 75.0-93.9%), baseline specificity 98.3% (CI 97.1-99.1%) and overall accuracy of 95.3% (95% CI 0.92-0.99). In both tests, some samples showed very low viral load and negative Ag-RDT. This disagreement may reflect the positive inability of Ag-RDTs of detecting antigen in late phase of infection. Among all cases with positive molecular test and negative antigen test, none showed viral loads > 10⁶ copies/mL. Finally, we found one false Ag-RDTs negative result (low cycle thresholds; 9 × 10⁵ copies/mL). Our results suggest that both Ag-RDTs showed good performances in detection of high viral load samples, making it a feasible and effective tool for mass screening in actively infected children.

Keywords: COVID-19; SARS-CoV-2; children; mass screening; rapid antigenic test; real time RT-PCR.

Figure 1

On the left (A), ROC curve for COVID-19 Ag FIA Ag-RDT, compared to real time RT-PCR on the right (B), ROC curve for AFIAS COVID-19 Ag compared to real time RT-PCR. On the x axis, 1-specificity; on y axis, sensitivity.

Figure 2

BOX Plot comparing cut-off index (COI) groups with viral loads. Horizontal lines represent mean, blue squares account for the interquartile range and vertical bars represent the lower and upper limits of values (range). Group 1, COI < 1; Group 2, 1 ≤ COI < 12 and Group 3, COI ≥ 12.