Accuracy of a nucleocapsid protein antigen rapid test in the diagnosis of SARS-CoV-2 infection

Diao, Bo; Wen, Kun; Zhang, Ji; Chen, Jian; Han, Chao; Chen, Yongwen; Wang, Shufeng; Deng, Guohong; Zhou, Hongwei; Wu, Yuzhang

OBJECTIVE: Rapid, reliable, and easy-to-implement diagnostics that can be adapted in early SARS-CoV-2 diagnosis are critical to combat the epidemic. SARS-CoV-2 nucleocapsid protein (NP) is an ideal target for viral antigen-based detection. A rapid and convenient method was developed based on fluorescence immunochromatographic (FIC) assay to detect the SARS-CoV-2 NP antigen. However, the accuracy of this diagnostic method needs to be examined. METHODS: This prospective study was carried out between February 10 and 15, 2020 in 7 hospitals of Wuhan and 1 hospital of Chongqing, China. Participants with clinically suspected SARS-CoV-2 infection were enrolled. NP antigen testing by FIC assay and nucleic acid (NA) testing by RT-PCR were performed simultaneously in a blind manner with the same nasopharyngeal swab sample. The diagnostic accuracy of NP antigen testing was calculated by taking NA testing of RT-PCR as reference standard, in which samples with cycle threshold (Ct) value ≤ 40 were interpreted as SARS-CoV-2 positives. RESULTS: A total of 253 participants were enrolled and 2 participants were excluded from the analyses due to invalid NP testing results. Of 251 participants (99.2%) that were included in the diagnostic accuracy analysis, a total of 201 participants (80.1%) had a Ct value ≤40. With Ct value 40 as the cut-off of NA testing, the sensitivity, specificity, and percent agreement of the FIC assay was 75.6% (95% CI 69.0%-81.3%), 100% (95% CI 91.1%-100%), and 80.5% (95% CI 75.1%-84.9%), respectively. CONCLUSIONS: With RT-PCR assay as reference standard, NP antigen testing by FIC assay shows high specificity and relative high sensitivity in SARS-CoV-2 diagnosis in the early phase of infection.