Objective: To explore the diagnostic value of serum severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid (N) protein assay in the early stages of SARS-COV-2 infection.

Methods: Serum N protein level in SARS-COV-2 infected patients and non-SARS-COV-2 infected population was measured by enzyme-linked immunosorbent assay (ELISA) double antibody sandwich assay. Colloidal gold immunochromatography assay was used to detect serum N protein antibodies in the above populations.

Results: Fifty cases of SARS-CoV-2 nucleic acid-positive and SARS-CoV-2 antibody-negative patients had a serum N protein positivity rate of 76%. Thirty-seven patients who were positive for serum SARS-CoV-2 antibody after infection had a serum SARS-CoV-2 N protein positivity rate of 2.7%. Serum N protein test results of 633 non-SARS-COV-2 infected patients, including pregnant women, patients with other respiratory infections, and individuals with increased rheumatoid factor were all negative, with serum N protein concentration <10.00 pg/mL at 100% specificity. Using SPSS 19.0 to calculate the receiver operating characteristic curve, the area under the curve was determined to be 0.9756 (95% confidence interval 0.9485–1.000, p < 0.0001), and sensitivity and specificity were 92% (95% confidence interval 81.16–96.85%) and 96.84% (95% confidence interval 95.17–97.15%), respectively. The best CUT-OFF value was 1.850 pg/mL.

Conclusion: The measurement of serum SARS-COV-2 N protein has a high diagnostic value for infected patients before the antibody appears and shortens the window period of serological diagnosis. It is recommended that the manufacturer establish two different CUT-OFF values according to the purpose of the application. One CUT-OFF value is used for the diagnosis of clinical SARS-COV-2 infection, and the other is used to screen out as many suspected cases as possible.

目的： 探讨血清严重急性呼吸系统综合症冠状病毒2（SARS-CoV-2）核衣壳（N）蛋白测定对SARS-COV-2感染早期的诊断价值。

方法：通过酶联免疫吸附法（ELISA）双抗体夹心法检测SARS-COV-2感染患者和非SARS-COV-2感染患者的血清N蛋白水平。胶体金免疫层析法用于检测上述人群中的血清N蛋白抗体。

结果：50例SARS-CoV-2核酸阳性和SARS-CoV-2抗体阴性患者的血清N蛋白阳性率为76％。感染后血清SARS-CoV-2 N抗体阳性的37例患者的血清SARS-CoV-2 N蛋白阳性率为2.7％。633例非SARS-COV-2感染患者（包括孕妇，其他呼吸道感染患者和类风湿因子增高的患者）的血清N蛋白检测结果均为阴性，血清N蛋白浓度<10.00 pg / mL为100％特异性。使用SPSS 19.0计算接收器工作特性曲线，曲线下面积确定为0.9756（95％置信区间0.9485–1.000，p<0.0001），敏感性和特异性分别为92％（95％置信区间81.16-96.85％）和96.84％（95％置信区间95.17-97.15％）。最佳CUT-OFF值为1.850 pg / mL。

结论：在抗体出现之前，血清SARS-COV-2 N蛋白的测定对感染患者具有较高的诊断价值，并缩短了血清学诊断的窗口期。建议制造商根据应用目的建立两个不同的CUT-OFF值。一个CUT-OFF值用于诊断临床SARS-COV-2感染，另一个用于筛选尽可能多的可疑病例。