Performance of a SARS CoV-2 antigen rapid immunoassay in patients admitted to the Emergency Department

Leli, Christian; Matteo, Luigi Di; Gotta, Franca; Cornaglia, Elisa; Vay, Daria; Megna, Iacopo; Pensato, Rosalia Emanuela; Boverio, Riccardo; Rocchetti, Andrea

OBJECTIVES: Given the ongoing emergency, there is a need to identify SARS CoV-2 infection in due course in various community settings. Rapid antigen testing is spreading worldwide, but diagnostic accuracy is extremely variable. We compared a microfluidic rapid antigen test with a reference molecular assay in patients admitted to the Emergency Department (ED) of a General Hospital from October 2020 to January 2021. METHODS: we included nasopharyngeal swabs collected in patients with suspected COVID-19 and in patients with no symptoms suggesting COVID-19, but requiring hospitalization. RESULTS: 792 patients of median age 71 years were included. With a prevalence of 21% we had: 68.7% [95% confidence interval (CI): 60.9-75.5] sensitivity; 95.2% (95% CI: 93.1-96.7) specificity; 79.2% (95% CI: 71.4-85.3) positive predictive value (PPV); 91.9% (95% CI: 89.5-93.9) negative predictive value; 3.8 (95% CI: 2.7-5.3) positive likelihood ratio (LR+); 0.09 (95% CI: 0.07-0.1) negative likelihood ratio. In the symptomatic subgroup sensitivity raised to 81% (95% CI: 70.3-88.6) and PPV to 96.9% (95% CI: 88.5-99.5) along with a LR+ of 32 (95% CI: 8.2-125.4). CONCLUSIONS: the new rapid antigen test showed an overall excellent diagnostic performance in a challenging condition such as that of an ED during the COVID-19 emergency.