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Diagnostic accuracy of a rapid SARS-CoV-2 Antigen Test among people experiencing homelessness: a prospective cohort and implementation study

De Nardo P, Tebon M, Savoldi A, Soriolo N, Danese E, Peserico D, Morra M, Gentilotti E, Caliskan G, Marchetti P, Cecchetto R, mazzariol A, Verlato G, Gibellini D, Tacconelli E

Introduction: Detection strategies in vulnerable populations such as people experiencing homelessness (PEH) need to be explored to promptly recognize severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) outbreaks. This study investigated the diagnostic accuracy of a rapid SARS-CoV-2 Ag test in PEH during two pandemic waves compared with gold standard real-time multiplex reverse transcription polymerase chain reaction (rtRT-PCR).

Methods: All PEH ≥ 18 years requesting residence at the available shelters in Verona, Italy, across two cold-weather emergency periods (November 2020-May 2021 and December 2021-April 2022) were prospectively screened for SARS-CoV-2 infection by means of a naso-pharyingeal swab. A lateral flow immunochromatographic assay (Biocredit® COVID-19 Ag) was used as antigen-detecting rapid diagnostic test (Ag-RDT). The rtRT-PCR was performed with Allplex™ SARS-CoV-2 assay kit (Seegene). Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were calculated as measures for diagnostic accuracy.

Results: Overall, 503 participants were enrolled during the two intervention periods for a total of 732 paired swabs collected: 541 swabs in the first period and 191 in the second. No significant differences in demographic and infection-related characteristics were observed in tested subjects in the study periods, except for the rate of previous infection (0.8% versus 8%; p < 0.001) and vaccination (6% versus 73%; p < 0.001). The prevalence of SARS-CoV-2 in the cohort was 8% (58/732 swabs positive with rtRT-PCR). Seventeen swabs were collected from symptomatic patients (7%). Among them, the concordance between rtRT-PCR and Ag-RDT was 100%, 7 (41.2%) positive and 10 negative pairs. The overall sensitivity of Ag-RDT was 63.8% (95% CI 60.3-67.3) and specificity was 99.8% (95% CI 99.6-100). PPV and NPV were 97.5% and 96.8%, respectively. Sensitivity and specificity did not change substantially across the two periods (65.1% and 99.8% in 2020-2021 vs. 60% and 100% in 2021-2022).

Conclusions: A periodic Ag-RDT-based screening approach for PEH at point of care could guide preventive measures, including prompt isolation, without referral to hospital-based laboratories for molecular test confirmation in case of positive detection even in individuals asymptomatic for COVID-19. This could help reduce the risk of outbreaks in shelter facilities.

Keywords: COVID-19 Ag rapid test; Diagnostic accuracy; Homeless; Vulnerable population.

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