Clinical evaluation of Sofia Rapid Antigen Assay for detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-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: This study was conducted in 2 community hospitals in Maryland from September 21, 2020, to December 3, 2020. In total, 2,887 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 predictive values, 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: For all patients, overall agreement was 97.9%; sensitivity was 76.6% (95% confidence interval [CI], 71%-82%), and specificity was 99.7% (95% CI, 99%-100%). We detected no differences in performance between asymptomatic and symptomatic individuals. As RT-PCR Ct increased, the sensitivity of the antigen assay decreased. The mean turnaround time for the antigen assay was 1.2 hours (95% CI, 1.0-1.3) and for RT-PCR it was 20.1 hours (95% CI, 18.9-40.3) (P < .001). No transmission from antigen-negative/RT-PCR-positive patients was identified.

Conclusions: Although 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.

Fig. 1.

(A) Box plot comparing cycle thresholds of RT-PCR assays between true-positive and false-negative results for the Sofia Rapid antigen assay. True-positive antigen results had a significantly lower cycle threshold on corresponding RT-PCR assays than false-negative results. (B) Sensitivity of Sofia rapid antigen assay compared to RT-PCR assay based on cycle threshold. Sensitivity decreases as cycle threshold increases. Thus, at lower viral loads, the likelihood of false-negative antigen test results is higher.

Fig. 2.

Box plot comparing cycle thresholds of RT-PCR assays between symptomatic and asymptomatic individuals. No statistical differences were detected between the 2 groups (P = .78).