Performance of a SARS-CoV-2 antigen rapid immunoassay in patients admitted to the emergency department

Abstract

Objectives: Given the ongoing pandemic emergency, there is a need to identify SARS CoV-2 infection in various community settings. Rapid antigen testing is spreading worldwide, but diagnostic accuracy is extremely variable. Our study 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: Nasopharyngeal swabs collected in patients with suspected COVID-19 and in patients with no symptoms suggesting COVID-19, but requiring hospitalization, were obtained.

Results: 792 patients of median age 71 years were included. With a prevalence of 21%, the results showed: 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+); and 0.09 (95% CI: 0.07-0.1) negative likelihood ratio (LR-). In the symptomatic subgroup, sensitivity increased to 81% (95% CI: 70.3-88.6) and PPV to 96.9% (95% CI: 88.5-99.5), along with an LR+ of 32 (95% CI: 8.2-125.4).

Conclusions: The new rapid antigen test showed an overall excellent diagnostic performance in a challenging situation, such as that of an ED during the COVID-19 emergency.

Keywords: COVID-19; Diagnostic techniques and procedures; Emergency department; Microfluidic analytical techniques; Reverse transcriptase polymerase chain reaction; SARS-CoV-2.

Figure 1

(A) Comparison of median CT value for molecular swabs corresponding with the 114/166 (68.7%) antigen-positive results with that for molecular swabs corresponding with the 52/166 (31.3%) antigen-negative results in the whole population. (B) Comparison of median CT value for molecular swabs corresponding with the 64/79 (81%) antigen-positive results with that for molecular swabs corresponding with the 15/79 (19%) antigen-negative results, in patients with fever and/or cough and/or dyspnea. (C) Comparison of median CT value for molecular swabs corresponding with the 26/54 (48.1%) antigen-positive results with that for molecular swabs corresponding with the 28/54 (51.9%) antigen-negative results, in asymptomatic patients. (D) Comparison of median CT value for molecular swabs corresponding with the 107/132 (81%) antigen-positive results with that for molecular swabs corresponding with the 25/132 (19%) antigen-negative results, considering positive molecular swabs corresponding with CTs ≥ 35 as low viral load samples.