LAUSR

Currently, the diagnosis of COVI-19 relies mainly on the real-time reverse-transcriptase polymerase chain reaction (PCR) assay. However, as examplified in a recent report in this journal, PCR assay results may be influenced by the sampling procedure and false-negative results occur [1]. Detection of antibodies to SARS-CoV has shown superior sensitivity when compared with RT-PCR assay [2] and the presence of IgG and IgM antibodies to SARSCoV2 has been reported [3]. We here describe the time course development of SARS-CoV2 antibodies and show that chemiluminiscence can be used as an effective method for the diagnosis of COVID-19. Between 28 January and 8 March 2020, 148 peripheral blood samples from 80 COVID-19 patients were collected, and the 487 samples from healthy donor were also included as negative control. Diagnosis of COVID-19 was confirmed with approved real-time RT-PCR assay targeting ORF1ab and N gene (Daan Gene, Guangzhou, China). Samples were archived at 80 C, and thawed at room temperature before analysis. This study was approved by ethics committee of Public Health Clinical Centre of Chengdu (No. PJ-K2020-01-01) and performed in accordance with the declaration of Helsinki. Because of the retrospective nature of the study, informed consent was waived. Detection of IgG and IgM antibodies was performed with chemiluminescence IgG and IgM detection kit (Orienter Biotech, Chengdu, China) according to the manufacture’s instruction. Briefly, samples were incubated with biotinylated SARS-CoV2 antigen, streptavidin-coated beads, phosphatase (ALP) labelled mouse anti-human IgG antibody, and buffer solution. After incubation, a beads-SARS-CoV2-Immuneglobulin complex will form. Beads were washed, incubated with ALP substrate AMPPD, and relative light unit (RLU) was analyzed with Access 2 immunoassay system (Beckman Coulter, CA). The signal-to-cutoff (S/CO) ratio was then calculated, values < 1 were regarded as negative, and values 1 were regarded as positive. Categorical data were expressed as numbers and percentage. Data were analyzed with SPSS 15.0 (SPSS, Inc., Chicago, IL). To test the diagnostic value of this assay, IgM and IgG to SARS-CoV2 were tested in serum from healthy control donors. In 189 healthy donors samples, none was tested positive, resulting in a specificity of 100%. Similarly, the specificity for IgG was 96.3% (11 out of 298 control samples were tested positive). Next we tested IgM and IgG in RT-PCR confirmed COVID-19 patient samples, and the temporal development of SARS-CoV2 antibodies is shown in Table 1. Within the first week, the positive rate of IgM and IgG was 22.2% (2/9) and 66.7% (6/9), respectively. The positive rate of IgM gradually increased to a peak of 77.3% (17/22) in the fourth week. The positive rate of IgG reached 100% at the third week, and this high positive rate remained afterwards. When either positive results of IgM or IgG was considered positive for COVID-19, the assay provide a sensitivity of 66.7% at the first week, and a sensitivity of 84.2%, 100%, 95.5% and 100% at the second, third, fourth and beyond 4weeks, respectively. In our study, IgM and IgG could be detected as early as 2 and 3 days after illness onset, and persist for up to 54 days. Two asymptomatic patients were analysed for SARSCoV2 antibodies. Both patients were isolated after contact with confirmed COVID-19 patients, but complained no symptoms during isolation. In both patients, IgG was positive at a high level (S/CO 107.82 on day 17 after