Clinical Evaluation of Sofia Rapid Antigen Assay for Detection of Severe Acute Respiratory Syndrome Coronavirus 2 among Emergency Department to Hospital Admissions

Abstract

Objective: To determine the utility of the Sofia® SARS rapid antigen fluorescent immunoassay (FIA) to guide hospital bed placement of patients being admitted through the emergency department (ED).

Design: Cross-sectional analysis of a clinical quality improvement study.

Setting: Two community hospitals in Maryland. From 9/21/2020 to 12/3/2020, 2887 patients simultaneously received the Sofia® SARS rapid antigen FIA and SARS-CoV-2 RT-PCR assays on admission through the ED.

Methods: Rapid antigen results and symptom assessment guided initial patient placement while confirmatory RT-PCR was pending. The sensitivity, specificity, positive and negative predictive values of the rapid antigen assay were calculated relative to RT-PCR, overall and separately for symptomatic and asymptomatic patients. Assay sensitivity was compared to RT-PCR cycle threshold (Ct) values. Assay turnaround times were compared. Clinical characteristics of RT-PCR positive patients and potential exposures from false-negative antigen assays were evaluated.

Results: Overall agreement, sensitivity, and specificity for all patients was 97.9%, 76.6% (95% confidence interval (CI): 71%, 82%), and 99.7% (95% CI: 99%, 100%), respectively. No differences in performance were seen between asymptomatic and symptomatic individuals. As RT-PCR Ct increased, sensitivity of the antigen assay decreased. Mean turnaround time for the antigen assay and RT-PCR was 1.2 (95% CI: 1.0, 1.3) and 20.1 (95% CI: 18.9, 40.3) hours, respectively (p<0.001). No transmission from antigen-negative/RT-PCR-positive patients was identified.

Conclusions: While not a replacement for RT-PCR for detection of all SARS-CoV-2 infections, the Sofia® SARS antigen FIA has clinical utility for potential initial timely patient placement.