Serological chemiluminescence immunoassay for the diagnosis of SARS‐CoV‐2 infection

Lijia, Shao; Lihong, Shen; Huabin, Wang; Xiaoping, Xu; Xiaodong, Lu; Yixuan, Zhu; Pin, He; Yina, Xu; Xiaoyun, Shan; Junqi, Wu

OBJECTIVE: Dynamic monitoring of the concentration variation of IgM and IgG in patients with SARS‐CoV‐2 infections and exploring their diagnostic value for coronavirus disease‐19 (COVID‐19). METHODS: A total of 15 patients with SARS‐CoV‐2 infection were enrolled as the COVID‐19 group, and 50 patients were enrolled as the control group. The concentrations of SARS‐CoV‐2‐specific antibodies (IgM and IgG) were detected by a chemiluminescence immunoassay (CLIA). RESULTS: According to the cutoff value recommended by the manufacturer (cutoff = 10 AU/mL), the sensitivity, specificity, Youden index (YI), positive predictive value (PPV), and negative predictive value (NPV) of IgM were 60%, 100%, 60%, 100%, and 89.29%, respectively; and 86.67%, 100%, 86.67%, 100%, and 96.15%, respectively, for IgG. We reassessed the cutoff value of IgM. When the cutoff value for SARS‐CoV‐2 IgM was 1.83 AU/mL, the sensitivity, specificity, YI, PPV, and NPV were 93.33%, 98%, 91.33%, 93.33%, and 98%, respectively. During dynamic monitoring of the concentrations of IgM and IgG in COVID‐19 patients, we found the shortest times before a patient became IgM and IgG seropositive after symptom onset were 1.5 and 2 days, respectively. The longest times were 7 and 8 days, respectively. The positive rates of SARS‐CoV‐2 IgM and IgG both reached 100% in 8‐14 days after symptom onset. CONCLUSION: The IgM cutoff value of 1.83 AU/mL for the diagnosis of COVID‐19 was much better than the cutoff suggested by the manufacturer. SARS‐CoV‐2 infection can be ruled out if antibodies against SARS‐CoV‐2 are still undetectable 14 days after symptom onset.