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

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.