Clinical significance of the serum IgM and IgG to SARS‐CoV‐2 in coronavirus disease‐2019

Abstract Objective To explore the clinical value of serum IgM and IgG to SARS‐CoV‐2 in COVID‐19. Methods 105 COVID‐19 patients were enrolled as the disease group. 197 non‐COVID‐19 patients served as the control group. Magnetic chemiluminescent immunoassay (MCLIA) was used to detect the IgM and IgG. Results The peak of positive rates of SARS‐CoV‐2 IgM was about 1 week earlier than that of IgG. It reached to peak within 15–21 days and then began a slowly decline. The positive rates of IgG were increased with the disease course and reached the peak between 22 and 39 days. The differences in sensitivity of the three detection modes (IgM, IgG, and IgM + IgG) were statistically significant. The largest group of test cases (illness onset 15–21 days) showed that the positive rate of IgG was higher than IgM. Also, the sensitivity of IgM combined with IgG was higher than IgM or IgG. IgM and IgG were monitored dynamically for 16 patients with COVID‐19, the results showed that serological transformation of IgM was carried out simultaneously with IgG in seven patients, which was earlier than IgG in four patients and later than IgG in five patients. Conclusion The detection of SARS‐CoV‐2 IgM and IgG is very important to determine the course of COVID‐19. Nucleic acid detection combined with serum antibody of SARS‐CoV‐2 may be the best laboratory indicator for the diagnosis of SARS‐CoV‐2 infection and the phrase and predication for prognosis of COVID‐19.

gold standard for the diagnosis of COVID-19. [3][4][5] However, SARS-CoV-2 RNA testing based on throat or nasopharyngeal swabs yields frequent false-negative. Many cases that were strongly epidemiological linked to SARS-CoV-2 exposure and with typical lung radiological findings remained RNA negative in their upper respiratory tract samples. 6,7 Therefore, it is of great necessary to use a rapid and accurate scheme of diagnosis based on different detection principles which could overcome the shortcomings of RNA detection.
SARS-CoV-2 antibody can be produced in COVID-19 patients who were infected SARS-CoV-2 for 3-15 days. 8 Therefore, antibody test of suspected COVID-19 patients could be a good way that reduced missed diagnosis when RNA testing is negative. Lijia et al detected SARS-CoV-2 IgM and IgG antibodies in [15][16][17][18][19] patients. They found that the shortest time was 1.5-2 days for the detectable antibody after symptom onset. 9 Ma et al 10

| Detection of SARS-CoV-2 nucleic acids
Pharyngeal swab samples were collected and placed into a collection tube pre-filled with 2ml virus preservation solutions. We purchased nucleic acid extraction and real-time fluorescence quantitative PCR kit from Da'an Biotechnology Co., Ltd, and Sansure Biotechnology Co., Ltd for non-COVID-19 patients, respectively. The experimental processes were operated following the kit instructions.

| Measurement of the IgG and IgM to SARS-CoV-2
Serum samples were collected using vacutainer tubes without anticoagulant, centrifuged after the blood completely coagulated, inactivated in a 56°C water bath for 45 min, and stored at −20°C. The SARS-CoV-2 IgM and IgG were tested by automatic chemiluminescence immunoassay analyzer, and the detection kit was provided by

| Statistical analysis
Statistical analysis was performed using SPSS software 22.0. Median (quartile) data were used for continuous variable, and Wilcoxon rank-sum test was used for comparison between different groups.
Countable data were expressed in percentage and analyzed by chisquare test. Take ɑ equal to 0.05 as the inspection standard. p ≤ 0.05 was treated as a significant difference.

| The demographic and characteristics of enrolled patients
There were 302 patients adopted in the present retrospective study,   Table 2).

| The properties of SARS-CoV-2 IgM and IgG detected by MCLIA
To evaluate the properties of the detection kits for SARS-  After approximately 0-7 days of experiencing symptoms, the titer of IgM and IgG antibodies gradually increased and began to decrease after 3 weeks ( Figure 2B).

| D ISCUSS I ON
After the detection of IgM and IgG to SARS-CoV-2 becoming "golden standard" for the diagnosis of suspected COVID-19 patients, The National Medical Products Administration of China approved urgently five antibody kits based on different principle of detection, some of them are used to test total antibody to SARS-CoV-2, the others to detect of IgM and IgG, seperatedly.
Some of the kits are for qualitative such as colloidal gold test, and the others are quantitative such as ELISA or chemiluminescent immunoassay. [12][13][14][15] In this study, we carried out the semi-quantitative detection of IgM and IgG to SARS-CoV-2 in 197 serum samples from non-COVID-19 patients by MCLIA to study specificity. Of the 197 non-COVID-19 patients including 234 serum samples (Table S3) Figure   S2). We also reviewed the medical records of these seven patients retrospectively, and these patients were also not treated with antibody preparations. We have not yet identified the reasons for false-positive IgG or IgM antibodies in seven non-COVID-19 patients. Our test results were basically consistent with the specificity given in the reagent instructions ( Figure S3). Taken 93.4% confidence interval as the standard for setting the reference interval according to the reagent specification, which means that 6.6% of the population is still not within the current reference interval, which may be one of the reasons for false positives. In

CO N FLI C T O F I NTE R E S T
The authors are declaring that there are no conflicts of interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
All data included in this study are availability upon request by contact with the corresponding author.