AUTHOR=Yokoyama Rin , Kurano Makoto , Nakano Yuki , Morita Yoshifumi , Ohmiya Hiroko , Kishi Yoshiro , Okada Jun , Qian Chungen , Xia Fuzhen , He Fan , Zheng Liang , Yu Yi , Mizoguchi Miyuki , Higurashi Yoshimi , Harada Sohei , Jubishi Daisuke , Okamoto Koh , Moriya Kyoji , Kodama Tatsuhiko , Yatomi Yutaka 

TITLE=Association of the Serum Levels of the Nucleocapsid Antigen of SARS-CoV-2 With the Diagnosis, Disease Severity, and Antibody Titers in Patients With COVID-19: A Retrospective Cross-Sectional Study

JOURNAL=Frontiers in Microbiology

VOLUME=12

YEAR=2021

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2021.791489

DOI=10.3389/fmicb.2021.791489

ISSN=1664-302X

ABSTRACT=<p><bold>Background:</bold> Several types of laboratory tests for COVID-19 have been established to date; however, the clinical significance of the serum SARS-CoV-2 nucleocapsid (N) antigen levels remains to be fully elucidated. In the present study, we attempted to elucidate the usefulness and clinical significance of the serum N antigen levels.</p><p><bold>Methods:</bold> We measured the serum N antigen levels in 391 serum samples collected from symptomatic patients with a confirmed diagnosis of COVID-19 and 96 serum samples collected from patients with non-COVID-19, using a fully automated chemiluminescence immunoassay analyzer.</p><p><bold>Results:</bold> Receiver operating characteristic analysis identified the optimal cutoff value of the serum N antigen level (cutoff index, based on Youden’s index) as 0.255, which yielded a sensitivity and specificity for the diagnosis of COVID-19 of 91.0 and 81.3%, respectively. The serum N antigen levels were significantly higher in the patient groups with moderate and severe COVID-19 than with mild disease. Moreover, a significant negative correlation was observed between the serum N antigen levels and the SARS-CoV-2 IgG antibody titers, especially in patients with severe COVID-19.</p><p><bold>Conclusion:</bold> Serum N antigen testing might be useful both for the diagnosis of COVID-19 and for obtaining a better understanding of the clinical features of the disease.</p>