Point-of-Care SARS-CoV-2 Antigen Testing As a Triage Marker for Subsequent Severe COVID-19 in Symptomatic Outpatients: A Multicentre Cohort Study

​Background: Symptomatic outpatients with COVID-19 and other respiratory infections present similarly, yet triage decisions often precede RT-PCR results. SARS-CoV-2 antigen rapid diagnostic tests (Ag-RDTs) provide immediate results and preferentially detect high viral burden. We assessed whether Ag-RDT positivity at presentation identifies patients who subsequently develop severe COVID-19. Methods: We conducted a multicentre cohort study of 36,020 symptomatic outpatients who underwent same-day Ag-RDT and RT-PCR testing at four Taiwanese sites from May to October 2021. Participants with RT-PCR-confirmed infection were prospectively monitored for 14 days; records of all participants were reviewed for repeat testing and clinical events, and the participating hospital system was the sole tertiary-care provider serving the study catchment area. Severe COVID-19 was defined as RT-PCR-confirmed SARS-CoV-2 infection followed within 14 days by invasive mechanical ventilation or in-hospital death. We evaluated triage and diagnostic performance and prognostic associations adjusted for the age-adjusted Charlson Comorbidity Index among confirmed patients. Findings: Among 36,020 participants, 306 (0·8%) were Ag-RDT positive, 254 (0·7%) were RT-PCR positive, and 36 (0·1%) developed severe COVID-19. All confirmed infections were detected at index testing; no severe event followed a negative index RT-PCR in available records. Ag-RDT positivity identified severe COVID-19 with 80·6% sensitivity (95% CI 64·0–91·8), 99·2% specificity (99·1–99·3), 9·5% positive predictive value (6·4–13·3), and 99·98% negative predictive value (99·96–99·99). Diagnostic sensitivity and specificity were 63·8% (57·5–69·7) and 99·6% (99·5–99·7). Among patients with confirmed COVID-19, Ag-RDT-positive participants had lower Ct values (median 19·5 vs 30·9; p<0·001) and a greater risk of severe disease (17·9% vs 7·6%); the ACCI-adjusted odds ratio was 2·90 (1·15–7·31). Interpretation: Point-of-care Ag-RDT positivity identified most subsequent severe COVID-19 events despite moderate diagnostic sensitivity. It may support immediate triage before RT-PCR results, but prospective contemporary validation is required.