COVID-19 N-Antigen



Molecular assays on nasopharyngeal swabs remain the cornerstone of COVID-19 diagnosis. The high technicalities of nasopharyngeal sampling and molecular testing, as well as limited reagent resources, limit our testing capabilities. Several strategies have failed, to date, to fully alleviate this testing process (eg, saliva sampling or antigen testing of nasopharyngeal specimens). We evaluated the clinical performance of the ELISA detection of SARS-CoV-2 nucleocapsid antigen (N antigen) in serum or plasma using the COVID-19 Quantigene® assay (AAZ, France).


Results were determined on 63 sera from 63 non-COVID patients and 227 serum samples (165 patients) from the French COVID and CoV-CONTACT cohorts with RT-PCR confirmed SARS-CoV-2 infection, including 142 sera ( 114 patients) obtained within 14 days of the onset of symptoms.


Specificity was 98.4% (95% confidence interval [CI], 95.3 to 100). Sensitivity was 79.3% overall (180/227, 95% CI, 74.0 to 84.6) and 93.0% (132/142, 95% CI, 88.7 to 97, 2) within 14 days of the onset of symptoms. Sera and nasopharyngeal swabs were collected from 91 included patients in the same 24 hours. Among those with high nasopharyngeal viral loads, that is, a Ct value below 30 and 33, only 1/50 and 4/67 were negative for N-antigenemia, respectively. Among those with a negative nasopharyngeal RT-PCR, 8/12 had positive N-antigenemia; The lower respiratory tract was explored in 6 of these 8 patients, showing positive RT-PCR in 5 cases.


This is the first evaluation of a commercially available serum N antigen detection assay. It exhibits strong specificity and sensitivity within the first 14 days after the onset of symptoms. This approach provides a valuable new option for COVID-19 diagnosis, requiring only one blood draw and easily scalable across clinical laboratories.


Antigen; Antigenemia; Blood; COVID-19; Diagnosis; Plasma; SARS-CoV-2; Serum.

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