Quantifying SARS-CoV-2 nucleocapsid antigen in oropharyngeal swabs using single molecule array technology

Dorte Aa Olsen*, Claus L. Brasen, Søren Kahns, Jeppe B. Madsen, Helene Kierkegaard, Henry Christensen, Anders Jensen, Thomas V. Sydenham, Jens K. Møller, Jonna S. Madsen, Ivan Brandslund

*Corresponding author for this work

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This study aimed to develop a highly sensitive SARS-CoV-2 nucleocapsid antigen assay using the single molecule array (Simoa) technology and compare it with real time RT-PCR as used in routine clinical practice with the ambition to achieve a comparative technical and clinical sensitivity. Samples were available from 148 SARS-CoV-2 real time RT-PCR positive and 73 SARS-CoV-2 real time RT-PCR negative oropharyngeal swabs. For determination of technical sensitivity SARS-CoV-2 virus culture material was used. The samples were treated with lysis buffer and analyzed using both an in-house and a pre-commercial SARS-CoV-2 nucleocapsid antigen assay on Simoa. Both nucleocapsid antigen assays have a technical sensitivity corresponding to around 100 SARS-CoV-2 RNA molecules/mL. Using a cut-off at 0.1 pg/mL the pre-commercial SARS-CoV-2 nucleocapsid antigen assay had a sensitivity of 96% (95% CI 91.4–98.5%) and specificity of 100% (95% CI 95.1–100%). In comparison the in-house nucleocapsid antigen assay had sensitivity of 95% (95% CI 89.3–98.1%) and a specificity of 100% (95% CI 95.1–100%) using a cut-off at 0.01 pg/mL. The two SARS-CoV-2 nucleocapsid antigen assays correlated with r = 0.91 (P < 0.0001). The in-house and the pre-commercial SARS-CoV-2 nucleocapsid antigen assay demonstrated technical and clinical sensitivity comparable to real-time RT-PCR methods for identifying SARS-CoV-2 infected patients and thus can be used clinically as well as serve as a reference method for antigen Point of Care Testing.

Original languageEnglish
Article number20323
JournalScientific Reports
Number of pages8
Publication statusPublished - Dec 2021

Bibliographical note

Publisher Copyright:
© 2021, The Author(s).


  • Antigens, Viral/immunology
  • COVID-19 Serological Testing/methods
  • COVID-19/diagnosis
  • Coronavirus Nucleocapsid Proteins/analysis
  • Denmark
  • Diagnostic Tests, Routine
  • Humans
  • Immunoenzyme Techniques
  • Nasopharynx/virology
  • Nucleocapsid/analysis
  • Phosphoproteins/analysis
  • SARS-CoV-2/immunology
  • Sensitivity and Specificity
  • Single Molecule Imaging/methods
  • Virion/chemistry


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