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

Clin Microbiol Infect. 2021 Feb;27(2):289.e1-289.e4. doi: 10.1016/j.cmi.2020.09.057. Epub 2020 Oct 5.


Objectives: Rapid, reliable and easy-to-implement diagnostics that can be adapted in early severe acute respiratory syndrome coronavirus 2 (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 10 and 15 February 2020 in seven hospitals in Wuhan and one hospital in Chongqing, China. Participants with clinically suspected SARS-CoV-2 infection were enrolled. NP antigen testing by FIC assay and nucleic acid (NA) testing by real-time reverse transcriptase PCR (RT-PCR) were performed simultaneously in a blinded manner with the same nasopharyngeal swab sample. The diagnostic accuracy of NP antigen testing was calculated by taking NA testing of RT-PCR as the reference standard, in which samples with a cycle threshold (Ct) value of ≤40 were interpreted as positive for SARS-CoV-2.

Results: A total of 253 participants were enrolled; two participants were excluded from the analyses because of invalid NP testing results. Of 251 participants (99.2%) included in the diagnostic accuracy analysis, 201 (80.1%) had a Ct value of ≤40. With Ct value 40 as the cutoff of NA testing, the sensitivity, specificity and percentage agreement of the FIC assay was 75.6% (95% confidence interval, 69.0-81.3), 100% (95% confidence interval, 91.1-100) and 80.5% (95% confidence interval, 75.1-84.9) respectively.

Conclusions: With RT-PCR assay as the reference standard, NP antigen testing by FIC assay shows high specificity and relatively high sensitivity in SARS-CoV-2 diagnosis in the early phase of infection.

Keywords: Antigen detection; Diagnosis; Fluorescence immunochromatographic assay; Nucleocapsid protein; SARS-CoV-2.

Publication types

  • Evaluation Study

MeSH terms

  • Adolescent
  • Adult
  • Aged
  • Antigens, Viral / analysis*
  • COVID-19 / diagnosis*
  • COVID-19 Testing / methods*
  • COVID-19 Testing / standards
  • Coronavirus Nucleocapsid Proteins / analysis*
  • Female
  • Humans
  • Immunoassay
  • Male
  • Middle Aged
  • Nasopharynx / virology
  • Phosphoproteins / analysis
  • Prospective Studies
  • RNA, Viral / genetics
  • SARS-CoV-2 / genetics
  • SARS-CoV-2 / immunology
  • SARS-CoV-2 / isolation & purification*
  • Sensitivity and Specificity
  • Young Adult


  • Antigens, Viral
  • Coronavirus Nucleocapsid Proteins
  • Phosphoproteins
  • RNA, Viral
  • nucleocapsid phosphoprotein, SARS-CoV-2